Tresspassing ticket for being on festival grounds Property Invasion, Damages, Trespass

Milwaukee, Wisconsin.

First, I am 21, I had been drinking but was not vulgar or belligerent. I was in a music festival (they sell alcohol in the grounds) and saw a man being hand cuffed. I went and talked to the police doing it and asked why they were and what this man had done. The asked me if I thought this was funny, cuffed me, took me to a gate and said "get out" and let me go. I bought a new ticket, went back in and was later grabbed by security and police again, taken in the back and was told I was kicked out once before, given a trespassing ticket and told if I entered again I would go to jail. They never told me not to come back in the first time though. Have I done anything wrong, how can they do this?