I assume since they are Bicycle brands, the rights all belong to USPC, it's possible the artists simply work there. Basing my assumption on the fact that the rights are written just inside the flap and only mention that they are trademarks of USPC.
-Yeah, looking at the actual Bicycle site (not USPC) there's no mention at all of artists. They have a series called the Anne Stokes Collection, so I guess they only give credit for featured artists. I suppose you could always contact them and ask directly though.