Whatever, Bob. I said in theory he could have put his ticket at risk. Pilots have been fined for a lot less, and one pilot had his certificate revoked because he didn't provide a map to the FAA on how to find his farm in Oklahoma, which had a rural route address instead of a named street address (he won it back after appealing to an administrative law judge). So, if you think the FAA exercises absolute reason when deciding what is and isn't actionable, great. That hasn't been my experience in dealing with fines and the mitigation process.
I have to say, when you get a hair up your ass, you throw on the blinders and refuse to look 5 degrees left or right of what you think is center.
So, I'm done arguing over something this meaningless. You're absolutely right that there's nothing in writing that prohibits someone from putting bumper stickers on a commercial airliner.
Feel better?