Because there a lot of annoying little shits that ride and make a bad name for all of us
to any parent that reads this, if your going to drop your kid off at the park, dont get out of the car, if your going to park and wait for him, dont come out and make sure hes alright, if he falls, wait a decent while or until someone approaches you telling you that your kid just fucked him self up riding (unless you never want your kid to have friends or seem some what appealing to others, do not rush to his or her aide every damn fall, let them learn from their own mistakes.)
seriously though the jist of all that is, just let them be.