Destruction is a huge part of street art. No one expects their piece to be permanent. There is a loose code as to when it is acceptable to go over someones piece and a good one can stand for a long time but it won't be there unsullied forever.

These throw ups are making a real statement and it is this:

Banksy is no longer a street artist. He's chosen to give that up by selling out and his work is worth less than a shitty throw up to those in the scene.