Well, he doesn't own the rights to the images (Game Freak / Nintendo do), but he does own the bandwidth. If he was asking for other webmasters to pay money for using the images, that might end in a lawsuit (once again, haven't indulged that much in law studies), but just asking for a link keeps it in the realm of fair use. PM blocking people from using the images is more of him preventing them from using the bandwidth than the images themselves, since, as you and sumwun pointed out, you could easily take the images from other sites or upload them to your own server.
As for your second question, I'm afraid I don't really know how often other sites take from the DB's gallery.