It is easy for them to get away with it. I contacted one company of seeing one of their designs on another site. They thanked me for letting them know, however, these infractions are occurring so frequently that they could not afford to take legal action against them all. So sad that these designers that put so much effort into their craft are being taken advantage of.
The reason is because it is lucrative. Customers can--and should--contact the copyright owners to share their concern. I have reported a couple of violators and know that at least one site was removed following my report (which likely was only one of many).