Fight the system yeah?
I'd venture it's not commerce but commercialisation he's opposed to.
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.
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.
The hollywood machine will pay for the noose that hangs themselves, as long as they make money in the next 6 months.
Making money and making your work available for mass consumption is not orthogonal to loving your work.