Because Banksy is a famous brand, and his work has big price tags attached to it.
The whole thing is totally unique and fascinating. Banksy has developed his concept so well. He clearly also acknowledges the absurdity of the situation. His self shredding painting is clearly a comment on the contradiction between the ephemeral and the valuable.
Street art is inherently ephemeral. It's the wild west. There are no rules (although sometimes etiquette), because by its very nature it is breaking the rules.
Seeing newspaper articles where people are talking about a Banksy piece having been "vandalised", or that someone cutting out a piece of their wall to sell it is "stealing something that belongs to the community".
At the same time, street art in general is viewed as vandalism. The law doesn't try to make distinctions based on the artistic merit of a specific bit of unauthorised public space modification.
But money and fame, that's something anyone can understand! If something is worth money, then it's good. You painted on my wall! How lucky I am! Oh yeah and the pictures great too, what a clever image!
I personally think his work is pretty good. But much more interesting is how society has reacted to it.