its not so much the whole suing thing as much as it just unappealing.
If you're trying to run a business, with clients and employees coming all day, the last thing you want is a bunch of kids hanging out on your front steps all day. It makes you look unprofessional, and it's an annoyance. If it's some professionals filming, and they're taking a few quick shots, then maybe it's not so bad, but when it's a bunch of kids hanging out all day...it becomes a pain in the ass. And these days there are skate parks everywhere, so you can't say they have nowhere to go. I live in the middle of nowhere, and there's 2 within 10 minutes of me.
the security guard was a dick, but i'm betting we aren't seeing the whole story. They'd probably already been warned. Definitely could've been handled better.
But regardless of how stupid a law is, it's still a law, and you follow it.