It is against US law to bring into the country any animal part that was obtained illegally. The law was meant to stop things like the ivory trade, but that's what I think they'll get him on.
However, I've read that Palmer was not able to bring any part of the lion into the US because everything was seized by Zimbabwe authorities. If that's the case, they'll get him for conspiracy to import.
Remember that old saying: If the Fed's want ya, they got ya.
Yep, Lacey Act, CITES. Gibson Guitar got in a lot of trouble over importing Rosewood allegedly in violation of one of these or similar acts also.