The prohibition on destroying evidence generally applies to civil litigation as well - but you're right, there were no legal proceedings that would have made the tapes protected evidence.
I seem to recall a couple of NY congressman making noises to Goodell about wanting to see the evidence, thinking they might make a push for criminal prosecution, and pffffttttt, evidence gone. We need signs in NO that say: IT'S THE HYPOCRISY STUPID. (I like B & B, by the way).