I can see that being challenged BIG TIME.
First off, because any Joe Schmo can buy a can of purple paint...AND any other color to spray over the purple.
Anybody can buy no trespassing signs too, or the materials to tear them down (in the same sense as painting over the purple). However, unlike typical signs, the paint can be painted over but not torn off completely. A paint over of it makes an attempt at tampering obvious.