Having anarchist sympathies, this is something I've given a lot of thought to. But basically, there's a difference between 'surviving' and 'selling out.' Banksy's gotta live, too. As long as he doesn't feel that he's had to compromise on his vision to release them, then I don't see any particular clash between being anti-commercial and selling some work.
There's always a tension between the purity of the message and how many people actually hear it. Banksy could stay totally anti-commercial, do no shows, publish no works... and just stay on the streets of London. Or, he could sell some works for 5 figures, do his art around the world, and have a larger number of people hear his message.
I see your point but the total value of the work he's sold over the past 5 years is in excess of £500,000 and that's not including books, documentaries and so on. In a country where the national average salary is under £25,000 he's hardly at subsistence level is he?
In terms of the message, yes he gets a wider audience but he gets a wider audience for a weaker message. When you're wealthy and Brad Pitt is buying your stuff (I think I'm remembering that right) being counter-culture and anti-commercialism doesn't really ring true.
Not saying I disagree with the decisions he's made, just that inevitably success will undermine his artistic position.
I'd venture it's not commerce but commercialisation he's opposed to.