Favre is always just having fun out there. Even when he's throwing dirty blocks, he's doing it with a smile on his face and just like a child. Favre, of course, doesn't even give a peek to Wilson or check to see if he's okay. As Mike Tirico put it, it's just "not cool" to throw a block like that, which is why it's a 15 yard penalty.
The other reason this is stupid is that Brad Childress called this play. He knew that Brett Favre may have a cracked rib and already has a tear in his throwing shoulder (not to mention the offseason surgery he had) but called a play that requires his 40-year-old quarterback to throw a block? Strange.