why do we consider kevin responsible for all that mess if Buzz provoked him?

We don't.

Kevin's stupid family does.

I think the purpose of that scene is that we're seeing it all from a child's (in this case Kevin) perspective, so his family appear far more unreasonable than they (probably) were.
Let's be honest, as kids we've all gotten in trouble for something we didn't fully understand and therefore it feels very unfair, but our perspective changes as we get older.

The reality is that because Kevin was in the process of throwing a tantrum, it's reasonable that he should be taken to time out.
One would hope, however, that offscreen Buzz is made to clean up most of the mess made as punishment for the provocation, and both Jeff and Frank severely reprimanded for throwing insults Kevin's way.

I always thought Kevin let his anger get the best of him when he pushed Buzz, that's what caused him to get in trouble.

I do feel Buzz deserved some sort of punishment for provoking him.

They probably sent Kevin upstairs to avoid upsetting Uncle Frank. He seems to be especially annoyed by Kevin even before the “look what you did you little jerk” incident, and the other adults probably don’t like Frank when he is angry.

Parents always have the "golden child" that seems untouchable, no matter what he does, and the one they blame for stuff, even if not responsible.

We see this in Home Alone 2 as well. They buy into Buzz's BS when he "apologizes" when it's clearly and get mad when Kevin speaks the truth.