This is an interesting discussion.
While the average skateboarder is probably not conceptualizing it this way and I myself don't necessarily agree with it, the appropriation of private property for ones own use, whatever that is... may it be skateboarding, sitting, graffiti, yarn bombing etc. can be framed as a political act defying the very idea of private property or at least looking to set more nuanced gradations along a public/private spectrum. Acts of this nature happen all over the city, people stroll around Harvard yard, sit on a stoop they don't own, teach their children how to ride a bike in an open parking lot, or plant a garden in a vacant lot. The list can go on and on. None of these are particularly destructive acts, which I clearly don't condone here, but they are all acts wherein people interact and negotiate with the very idea of public/private. I think at its base that's what this discussion is about; how various agents with competing interests and varying ideas of and respect for the simple mine yours distinction navigate their environment.