Well for being against capitalistic culture, the Banksy movement acts pretty capitalistic...I guess it's not about making a well-deserved profit, but more about being incoherent with your message.
Really at this point it has highlighted it is impossible to be mainstream and maintain the status quo. When you start putting out unique works on random buildings and people are willing to pay enough to cut out the wall and replace it for a windfall it is fundamentally impossible. The subversion is fundamentally lost. Even doing something akin to putting Gitmo in Disneyland to force people to confront crimes committed in their name while trying to escape would see it scavenged and sold. Given a choice between making random landowners richer and himself it becomes clear why.
Imo you can be against capitalism, but you still live in a capitalist society and need to somewhat conform to it in order to live. It's not like Britain is going to become communist any time soon.
It is not, but their whole act has been street art, and purposely making their art hard to obtain, and implying an anticapitolist bent. However its all kind of an act now, with hard to obtain art just raising the percived value. Brilliant, but contrary to the image many have of them.