Again, you are misunderstanding the intention. 9/11 was an act of terrorism. Setting police cars on fire is vandalism, destruction of government property, maybe something for endangering police officers or something. All things condoned by your local friendly "anarchists".

What it isn't is terrorism.

Now go and listen to some Rage Against the Machine. Are they terrorists?

Is lighting crosses on fire terrorism?

Yes, because the whole point of that act is to terrify whoever lives on the property of the cross you're burning

Why are people lighting cop cars on fire? Please don't say it's to make an intelligent and nuanced political statement.

Because they want to destroy the government. Or their frustrated. Or mad. Or feeling mischevious.

None of those things are terrorism

Calling lighting some cars on fire terrorism sounds more frightening than anything else. I feel terrorized just by that.