I agree that
1) a response is not required for a save the date
2) if you are positive you will not attend, it is extra considerate to let the hosts know that so that they can plan better. And if your own sister's wedding or your transfer to Tokyo get called off, you can always call and say "Good news -- I'll be able to be at your wedding after all," as long as it's before the invitations would have gone out. By the same reasoning, if you are sure you will come (as sure as anyone can be, of course -- you don't know if you'll get sick or something), nice to let them know that, too. We have already gotten a couple of surprises regarding people's plans in both directions for our daughter's wedding this summer after the magnets went out. (The magnets didn't say anything about responding, but some people did anyway.)
And I would add that whether or not you know your plans, it's nice to let people know you got the thing and share their excitement. My daughter is so happy anytime anyone sends an email or text saying something like "We got the magnet! It's adorable. They look so much in love. " You will really warm their hearts.
Oh, and if they have a wedding website where you can leave a greeting, that also makes them really happy.