Like acroy said, it is voluntary. You can do it too. Find a site you like and donate. I've given to carbonfund.org. There are many other options if you are concerned about the sequestering problem with trees.
I know that riding a bike is an awesome way to reduce carbon, and that a lot of people worry about fraud with carbon offsets. But I decided to give anyways because I've started to believe that doing nothing at all is worse than getting ripped off trying to do the right thing.
There are still plenty of other things we can all do if anyone doesn't believe in offsets.