Is writing on a car's back windows illegal?

Answer 1

Provided the writing does not greatly diminish the ability of the driver to see out of said window, no, it is not illegal. What determines such ability is fairly inconsistent, but if you can't clearly read the license plate of a car behind you, then it's too much.

In the United States, that law is determined by the state the vehicle is registered in. And only enforced by authorities within that state.