The guy claimed he was doing it as a joke. Favre zealot or pratical jokester? You make the call after reading this part of the story, The driver told police that he took the sign as a joke after telling co-workers he was going to heist the placard, Hellmann said. The driver also said he knew the sign was to be donated to charity and suggested that he was going to take it to a charity himself after he was done with the sign. He was unable to name a specific charity, Hellmann said. 

Although, I would side with just a guy hopped-up on liquid courage. Accordin to the article, The suspect smelled of alcohol but passed roadside tests, Hellmann said.