It may be because, at some places shelters and pet shops do cooperate. They may together an exhibition say every thursday afternoon, where representants from the shelter come and do exhibit the adoptable cats they do have at the moment.I'm curious about the context in which you are using the term Petshop. This just came up in another thread recently where the poster said she had gotten her cat from a petshop. Once it was fleshed out it became clear she meant she got him from a shelter/other type of rescue organization. It's just where she lived that was the term used.
So, the cats are actually from the shelter, but the meeting place is the easily accessible pet shop.
The cats being there just a couple of hours at a time.
Often there is also a vet at the same time.
Such an solution works surely OK. A lot of people, perhaps would never go to the shelter (these are often outside the citys centrum), but they can go to the pet shop, which usually IS in the citys center. So more people do see them, and more potential adoption homes are exponed this way.
Its unfair for shy cats, and most unfair for ex-semiferales, but perhaps you cant get everything.
Otherwise, like EB24 said, as the rule of thumb, we do hang a big warning sign for buying your cat or dog from a petshop.