Petco is far from perfect as a company, but the little paper tag (you can see the bottom right corner in your photos) does let customers know those fish are not for sale at the moment, and the protocol is to cap the bulkheads so the fish are isolated for treatment. I've worked for Petco, and I've worked for very nice private-owned LFS's. All fish stores get sick fish in, it's unavoidable. They come from the suppliers, sometimes it is in the gills and you can't see it at all until a few days or a week later when the spots appear. It seems this petco did isolate the fish, so I'm not sure petco has really done anything wrong here