They appear to be violating their own code of behavior:
Q: What About Lost Pet and Garage Sale Signs?
A: In most locations these are illegal signs. Some cities have zero tolerance and remove these signs. Many sign sharks leave these signs alone, especially if they look non-commercial and relate directly to the immediate neighborhood. Also, these sign typically are only up from Friday through Sunday. The major difference is that these are not commercial enterprises, but simply a neighbor trying to get rid of some junk that another neighbor may need. For these reasons, CAUSS does not promote the removal of these signs. Lost pet signs are in the same category
http://www.causs.org/faq.html