Is kicking a fumble legal?

[PackersFTW]

I saw a highlight of the Bengals game, where the snap went over the punter's head, and the punter ran after it and kicked it out of bounds so that the other team didn't recover it and run it in for a touchdown. I would have figured this to be illegal somehow, otherwise you could just kick it out of bounds after your team fumbles so that you retain possession. Maybe because it was 4th down it didn't matter? Still wouldn't there be some kind of penalty? (Could have been one, I don't know.) Still, it got me thinking about kicking fumbles (out of bounds or not) and all the rules that surround it.