Do you think it is impolite to press 'agree' on purpose (not by accident), when you actually disagree, and state that you pressed agree just to be polite and so on, on Amirite?

In general if I disagree with something I just move on and don't bother with downvoting or disagreeing. If I like something (because it is clever, insiteful, or witty) I will agree or like it even if I don't actually agree with the content.

Someone did that to me

I think it's up to the responder to decide what to press.

I don't think it's impolite. I don't know what to call it, but impolite is not the word I would use to define it.