Destroying property is a crime.
There's a difference.
Strictly speaking, the act of personal locomotion on a four-wheeled board is not a crime in any jurisdiction in the United States. However, many cities have outlawed skateboarding on public edifices because it results in destructive grinding and causes injuries, the costs for both of which are borne by the taxpayer.
Furthermore, the culture associated with skateboarding youth is widely held to be a celebration of delinquency: marijuana use, rebellion against authority, truancy, and other problems are known to be prevalent in these juvenile populations. While this is no reason to discriminate against skateboarding in general, it sure doesn't help the image of the activity in the public's eye.
Police Officer: "Young man, did I just see you skating down that marble staircase?"
Boy: (stares sullenly at ground) "Skateboarding is not a crime."
Officer: "Take those headphones off while I'm talking to you. Now what does that sign say right there?"
Sign: "Skateboarding on Town Hall property is subject to max. fine of $250 or 90 days in jail by order of City Rev. Code 08-2776."
Boy (fidgeting): "I dunno. Skateboarding is not a crime."
Officer: "And what's this? Did I just see a dime bag fall out of your pocket?"
Boy (crying): "Skateboarding is not a crime."