pawsplay
Hero
No, it probably would *not* fall under fair use. On that same measure, it is true that if a company doesn't defend its IP it can potentially lose said IP. However, a company can also broadly license usage of its IP or declare that it will not prosecute certain violations without jeopardizing its IP either. Paizo could lay a blanket statement down that says, "We authorize the use of our IP in publication so long as that usage does not infringe on commercial uses of said IP." With that, they wouldn't need to send C&Ds, would have a protected IP, and could still send C&D to people that actually abuse the IP in a way that is detrimental to the company. (Now, if they deemed this was actually detrimental to the company, then more power to them--they have the right to defend their IP however they choose).
A court might find that licensing digital versions of those cards to users, or the OP site in general, is a commercial use that should be protected.