Jimmy Carter did it too, I think he used duck hunting like Clinton did.
Kerry seems to be an avid hobbyist, everything from cycling to hunting. It's because:
1: He wants all voters to think he's "one of us".
2: He has a full time job that doesn't require his attendance, and has nothing better to do since he quit showing up at work almost two years ago. (not entirely true, he did show up in DC last year to vote for gun control a few times, it was the only topic he bothered to vote on in 2003)