Just because some rich people feel like attaching monetary value to most art doesn't mean Banksy's art has to fall in line. It's not like you're going to take down a section of wall and sell it, and someone did, he would also be missing the entire point.
Just be happy that cameras exist and take a picture.
[1] http://www.independent.co.uk/incoming/article8886934.ece/ALT...
http://www.nytimes.com/2013/08/14/arts/design/another-banksy...
As for missing the point, I'll raise you death of the author. A piece of art must stand on its own merits, not the intentions of its creator.
(Any measure of art preservation is an inconvenience to viewing it, but we generally consider this a worthwhile tradeoff)
Yes, generally we do, and I'd agree for graffiti done in canvas or similar mediums, but I think street art is an obvious exception, since the canvas is the whole street. You could only preserve the artwork by preserving at least the whole building, and possibly more than that.