Plastic Jesus - Why Can't a Wall Just Be a Wall?

Plastic Jesus interacts with this building in Los Angeles.

The 'Advertise Here' stencil was already there.  Then Plastic Jesus came along with another stencil that seems to taunt the wall.  It asks, 'Why can't a wall just be a wall?'.

Indeed.  Why can't a wall just be a wall?  (With street art on it?)