The idea is that if the yard can't unload them fast enough and the police do a surprise inspection, the orange paint (and etching the VIN on the cat is another common strategy) on some cats on the shelves makes it look suspicious, and if they can't explain where it came from, it can get legally rough for the yard. So, ostensibly, some of the dirty scrap yards will refuse to buy cats that have been marked like this. Edit: Or, at least, without more detail and proof about the origin of it.