Oh you are more than welcome to voice your opinion in an appropriate manner, there is a difference between giving an opinion/advice and raging at somebody.
Many times I've seen a build and thought "tf are you building? " but I haven't said it like that, instead I've gone "hey Lulu did you not build ardent for a reason? Just curious as your build isn't something I've seen before and wanted to know if it worked better than an ardent build. " in the end she told me she forgot about ardent that's why she hasn't opted for that build. We had a discussion about it just fine without either getting heated because I wasn't harassing her about her build I was inquiring instead.
But most people don't do it that way with most people it's "your a fucking support build sightstone. " "what the fuck are you building you trash" they basically have a "I don't care what you want to build you will build what I say" can of attitude and while yea a support should build sightstone nobody has a right to harass, insult or rage a person because they haven't done so.
That's what make it unacceptable to you, isn't it. Therefore the "telling people how to play" part is unrelated to all this and could be replaced with anything else.
It is still about telling people how to play though, even if you give advice on what to build they don't have to take it and nobody has a right to force them because they can build what they want, you can advice then but you can't actually tell them how to build/play.
Well actually that's what makes it unacceptable to riot which is why so many people get punished because they don't just offer advice they rage.
The same goes with in game, just because you don't like how somebody is playing doesn't mean you have a right to tell them how to play instead you can give them advice but ultimately they're allowed to play how they want as long as they're not breaking riots rules and trolling for example.