And people who think so can opt-out obviously. Unless you think organ donations as a whole are a bad idea, opt-out is the right thing to do.
I actually do not necessarily agree with your last sentence. I think that organ donations as a whole are a good idea, but I don't think takings are a good default for all cases. I'm not sure about this case, partly because of the sticky issue of ownership and transfer, which I won't bother going into unless someone asks.
Yesterday I heard an interesting story on the radio about US President Obama's pick to head the Office of Information and Regulatory Affairs, Cass Sunstein. I recommend checking out the story, but here are a few key excerpts.
At the risk of starting a discussion that will be wrecked by political wrestling, I'm always hopeful when I hear about governments applying what we learn from science to policy. Not to say that this always generates good policies, but it does generate the best policies we have reason to believe will be good (so long as you ignore the issue of actual politices that might get in the way).