Thinking about it more...Goodell strengthened the personal conduct policy so that it's no longer just about committing crimes:
"It is not enough simply to avoid being found guilty of a crime. Instead, as an employee of the NFL or a member club, you are held to a higher standard and expected to conduct yourself in a way that is responsible, promotes the values upon which the League is based, and is lawful."
That said, I don't believe any action has ever been taken in a situation where there was no suggestion of illegal activity. What Spikes did is seriously stupid for a guy in the public spotlight (and with a lot of recognizable body art). But it was just some exhibitionism for a community of consenting voyeurs.
Penalizing a player for a more "crime against good taste" would be crossing a huge line. Compare Spikes' "values" offense to, say, being found delinquent on child support to your boatload of baby mamas.