If I may deter from the bandwagon for a second and perhaps shred perspective to the other side.
You were practicing on a playground while parents and kids were around. notably kids. Young kids will see you and see what you are doing and want to imitate that. That's not a bad thing necessarily, because it is a cool thing to be able to do. But because they're just kids they haven't been taught how to properly do the things you're doing and could end up really hurting themselves. Now put yourself in the parents' shoes. You don't want your kids seeing this guy mess around on the playground and then have the kids start trying to imitate you and hurt themselves; also if you're practicing stuff like that people aren't going to want to mess with you and talk to you because they think you'll hurt them or get in their face or something. So what do you do when someone's bothering you and you don't want to get into conflict yourself? Call the cops.
Is it fair? Probably not. Is it justified? Well, I just justified it. Maybe before you scream 'victim' you should've considered that what you were doing had potential of hurting others, even if you didn't intend it. Before you label the parents' actions as blind ignorance, maybe you should've realized that doing backflips off the top of the twisty slide for all the little kids to gawk at and try to imitate may have been just as ignorant.