Is it legal to hang out on parking structures?

So I'm a 14 year old boy living in ann arbor mi and I like to hang out on the tops of parking structures to get a good view of town and I also really like the wind in my face. I started liking to go up on them when I was a little kid with my mom or dad, then as I got older and started walking around town by myself I started going up there by my self, I always went and hung out there wile my mom was in church. So then just last night I was going to The Ann Arbor Summer Festival with a few friends for my birthday, and we were there for a wile, then we went down to the river, then they went back to the festival and I went to go hang out on a parking structure. No sooner had I stepped out the door to the top, three university police oficeres waved me over, asked me what I was doing there, I said I was just coming up there to hang out and they said that was not allowed, so I was like Ok I'll leave then. And then one Of them saw the 2 inch pocket knife that I always carry with me and then they freaked and were like "sire take that out and set it on the ground" and then they searched my pockets and had me write down my name, date of birth, and number, and my dads number, then they told me to beat it and didn't give me my knife back (granted I was so flustered that I didn't think to ask for it back but still they can't just take it write?)