U.S. Postal Service Bashing Presidents?

I’m not exactly sure what to make of a quasi-government entity that bashes past presidents in such a manner.

Jonathan writes:

I took a pic of a U.S. Mail truck while I was working. Just in case you don’t look at the picture it’s the rear of a U.S. Mail truck with a silhouette of a man bent over with a gas nozzle shoved up his rear and the words “Thanks George”. Now the U.S. postal service is bashing an ex President?

The closest I could find online to the above sticker was this on Cafe Press:

So when did it become acceptable for government vehicles to publicly bash past presidents – and would we hear a bigger outcry were this a “Thanks Obama” sticker on a government vehicle? (Rhetorical, I know.)