Because they don't like it.
You made the correct observation. People are misusing and, funnily enough, abusing the word "abuse". If they don't like something, if they lose to something, if something isn't fitting their incorrect, preconceived notions of what you are supposed to do (e.g. "The Domination Tree is the Assassin Tree.") they can just call it "abuse". Then you don't need a reason, "abuse" makes it sound bad, so that becomes the reason the game should change things.
There's that argument that it isn't being misused by people, where he argues for situations where it could be correct. But we all have seen the threads, where people say it's wrong that AD Casters are building AD Caster items or that these champions shouldn't be allowed to use the "Tank" tree despite it being self-evident that they are allowed.