Because in semi-public places, like a store, you are only trespassing if you've been told to leave (you are trespassed).

Why "are you trespassed" if you're told to leave? I understand that you might not be engaging in the act of trespassing until you have been made aware that you must leave. But refusing to do so surely then means that you are trespassing not that you "are trespassed?

Trespassing is the act. The trespasser is the subject undertaking the act. The object that is being trespassed upon is surely then the offended location and/or person?