I mean if I buy talisman of the Ascension, the game actively blocks me from buying spellthief edge. Pretty sure its the same with the machete based stuff, yet once I get say a ninja tabi, I am free to buy another pair of moba boots.
Isn't there some easy way to make this impossible seeing as there are only two instances where somebody would do this: Accidentally, or trolling?
Gotta get them passives, son
The reason you're blocked from spellthiefs is because you're not allowed to stack gold gen items anymore. Excluding avarice.
Isn't it obvious? Riot plans to release a champion soon who utilize boot based abilities. The core of his build will be multiple boots and generally moving forward multiboot builds will become viable for certain champions.
Well, I mean, you really should have one for each foot. Hobbling around the rift with just one is kind of weird
Most Champions have 2 feet. You need a Shoe (or Boot) for each foot or else your champion will hobble around awkwardly and be very uncomfortable which will lead to missing their skillshots. 
They have tried to make the support gold-generation items and jungle items very cost efficient; they don't want you to buy more than one.
They don't care about doing the same for boots, since the MS doesn't stack.
This OP 1 Riot 0
it makes perfect sense too.
One can never have too many hats
Because a lot of boots have generally useful passives... Sure, not enough to warrant buying multiple boots, but there were times where I kept my early/mid game BoM and bought Mecury treads later because I needed the tenacity. Ultimately, I will sell BoM then again, but not necessarily right away. The short version is: Because while it's not an optimal strategy, it's not completely wasted and is also not overpowered.
Gp5 and jungle items are limited for balance reasons, more than anything else; and the reason you can't buy multiple basic boots is because the second pair literally gives you no stats at all. (Why you can buy basic boots and upgraded boots is probably a technical reason, since you couldn't buy another pair of upgraded boots without buying the basic first).
It's likely something that was overlooked over the years. With any luck, riot might read this and bring it up during their pre-season changes meeting.
gotta have dem hand boots ( ͡° ͜ʖ ͡°)
Well you see, you can gain different unique passives from different shoes which stack. For example, will provide auto attack damage reduction, and will provide tenacity. Therefore, if we do not care about any stats other than those provided by these unique passives, and wish to gain these passives with the least gold cost possible, it is theoretically possible for it to be wurf to buy two pairs of shoes and dangle the extras around your neck as a stylish new fashion.
Furthermore, you can enchant each pair of boots separately, giving you access to such OP combinations as 
It's only fun if they run http://s3-ec.buzzfed.com/static/enhanced/webdr02/2013/1/17/13/anigif_enhanced-buzz-5230-1358449084-5.gif
I want to build 4 pairs of boots on 
. How does it help when only two of her legs are faster then the other 6??? :(
you have 2 feet don't you? I always buy 2 boots, just makes sense. Why does everyone think that a boot purchase is two shoes, it only shows 1...
Maybe you want to stack Sorc Shoes for all the Mpen?
Same reason why the game allows you to get 
instead of forcing you to go 
I guess.
Made even more sense before the AP item nerfs.
Your argument is valid as the extra M pen, AR, MR or AS can be useful, but in highly specific scenarios. The extra 15 M pen is, as far as I can tell, only useful against heavy MR/low HP builds. However, every Mundo worth his doctorate would get some HP as well, so it's highly unlikely the added stats of having 2 boots of the same kind will ever have a usefulness outweighing a different item.
Simplest solution: make every boot stat a unique passive, same as the rest of them. Though personally I prefer the freedom of choice.
Or because some people got size 15 feet and needs a pair just to make one complete shoe - duh!
Should the game do this with any item with a unique passive where you already own an item with the same unique passive (not just boots)?
How about items with CDR when you are already over the CDR cap?
Hey now, the Dr. Mundo 6-boot build is amazing.
I need more hats
