They do sell full rights for an extra $199:

"All logos cost just $99, or if the logo has not been purchased before, you can buy it exclusively and own the full rights to the logo for an extra $199. "

I can see how this would be really attractive to many businesses; after all, most people who don't interact with designers on a regular basis view a logo as a very secondary matter, and if they can get a unique one for $300, that's probably good enough for them.

These people are probably crummy clients for any designer anyhow, so I don't feel like designers are really losing out. Heck, it might even be a worthwhile place to sell extra materials (e.g. logos that were not accepted).