I mean honestly it turns an already power buildable heal into an even more massive one. It's pretty much the sole reason that assassins have to be able to 1 shot squishes now, and there is already cases where it doesn't apply the bonus physical damage as lifesteal 
I totally agree that AD Carries are in need of serious changes. Not necessarily in terms of that they give an unfair advantage to one team, after all they can still be bursted and each team gets one of them, so it's fair, but that there are so little options to outplay them other than straight up killing them, which really narrows down the pool of viable champions to those that are AD Carries and those who can kill AD Carries. This would be one interesting idea that needs testing during pre season, but I don't think it's quite enough, since the Melee vs. Range interaction is flawed.
I'd go one step further and say that AD Carries are not broken, they don't need fixing, what they are doing is exactly what they were made for. After all Riot gave multiplicative scaling to arguably the strongest stat in the game (Range) and it's their identity to dish out tons of damage by automatic attacks. There's no way of changing them without taking away most of their core strengths.
My suggestions to Riot are:
Remove the slow from red buff on ranged champions. An AD Carry with an Escape shouldn't have a slow. No matter how much you nerf the true damage on it, it's still the slow that makes it broken on ADCs. Ashe should be one of the few ADCs to get a slow, along with Kog'Maw, MF, Varus and the likes.
Remove, or seriously impede, the ability to kite in teamfights. At the moment, you don't give up on DPS when you kite backwards, which makes no sense whatsoever. Dota2 has turning speeds for that, but I don't think that's the solution. In my opinion, an ADC should position at some place and start shooting, Twitch isn't supposed to kite a Riven to death (Doublelift vs Megazero anyone?), if he repositions, that should happen with a dedicated spell like Ezreal E, or in combination with a serious lack of DPS while doing so.
Fix Last Whisper. This item was supposed to be an anti-tank item, not the go-to item in every single game. Make it penetrate %bonus armor! Alongside with that, it is necessary to address the Crit / Lifesteal interaction, as mentioned in this thread. %Damage based sustain attached to multiplicatively scaling damage, who on earth thought that this would be a great idea?
AD Carries are great at pushing turrets, clearing waves, taking Dragons / Barons, while also being absolute beasts in teamfights. So if you engage against AD Carry you are just as bad off as you are when you don't, since he'll be great in any possible situation come late game. I don't think that their role should excel at everything. Make them weak at pushing turrets, or taking objectives, or clearing waves, or poke, or teamfights. But don't let them be the center of everything. The fact that there is a class designed to keep them safe, one to distract the opponent's AD Carry and one to kill said ADC is just sad.
Yes, there is going to be an outcry among AD Carries for taking some of their strengths and adding meaningful weaknesses, I am sure DL will say once again that there's no reason to be AD Carry main now and Tabbz will say on facebook that Supports should go back to being a wardbot, but think of all the power you could give them in their strengths once you have defined what an AD Carry should not be good at. As of right now, they have the best itemization, the highest damage, an entire class dedicated to make up for any potential weakness (did it ever occur to you that it's not normal to see Mikaels Crucible every single game?), in-combat sustain attached to Damage dealt (either you burst them, or…), high mobility, a permanent slow and the highest range in the game. The only reason this is balanced and doesn't show in winrates or anything like that, is because every team picks one of them, keeping the winrate of ADCs at about 50%, with the current FOTM at the top and the recently nerfed at the bottom. But nerfing individual ADCs doesn't solve the problem, it just pushes other champions to the top. The class as a whole is inherently flawed, and if Riot intended to fix them, they had to start from scratch.
It's an interesting idea. Here's some thoughts:
Lifesteal is currently maxing out at about 20% given some builds. Most ADC builds will have either or 
which give give most ADC's a mid game 15-20% lifesteal total. Let's just say 20%.
Most builds have about 300 flat AD with 
giving the crit a total addition of 150%. This crit is usually every 2nd AA.
Maths:
300 * .2 = 60 hp
750 * .2 = 150 hp
Make that an average for each AA (shared work maths):
60hp * 45% = 27
150 * 55% = 82
27 + 82 = 109 avg hp per AA
So, without crit factored in, each AA returns 60 hp. With crit factored in, each AA returns 109 hp.
Having said that, here's the real problem with Crit: It does crazy damage to everything except buildings. That 
is nuking you for 750 AD and 300 AD every second for about 1000 damage every second: do you really care that she's getting 200 HP back every second with crit factored in, or 120 without it factored in? That's a WHOPPING difference of 80 hp. If you can't do that kind of damage to her anyway, you got bigger concerns.
You could just as easily ask why attack speed gives life steal. The two stats are functionally identical.
Life steal is applied by auto attacks. If you get bigger auto attacks, you get bigger lifesteal. It'd be a bit odd if only some AA multipliers worked with LS and others didn't.
Well for starters to get a crit you normally have to basic attack, Amd life steal applies to basic attacks, and not all squishes suffer from this. If you play teemo and a yi full build comes to you Q him right after his Q and you can kill him in 2 seconds because of teemos blind.
Muramana and BotRK both apply lifesteal. Assassins always had to be able to insta-gib squishies, that's literally their entire job. Carries are supposed to out-scale everyone, that's literally their entire job.
http://leagueoflegends.wikia.com/wiki/Life_steal http://leagueoflegends.wikia.com/wiki/On-hit_effect
Life steal is an offensive stat that converts a percentage of the physical damage dealt by a champion's basic attack or ability that applies on-hit effects to health regained.
Life steal applies to all physical damage dealt on-hit, including any sourced from on-hit effects (with a few exceptions). Sheen item.png Sheen and NasusSquare.png Nasus's Siphoning Strike.png Siphoning Strike is a renowned example of powerful sustain as life steal not only applies to the basic damage on the attack, but the bonus damage on Sheen and the stacked damage on Siphoning Strike.
Life steal is an offensive stat that converts a percentage of the physical damage dealt to health regained. A champion's basic attack, abilities that apply on-hit effects, and abilities that modify your next basic attack all benefit from lifesteal.
You are provably wrong by a single look at the Wiki.
it is really a good question!
