Unfortunately for those activists, Obama cannot repeal DOMA by himself. There are still something like 40 states that ban gay marriage. There are still a number of moderate Democrats who really, really do not want to vote on this issue, because it might cost them their jobs.
The genius of a move like this is that it actually does some good, but doesn't really give the right enough to take him on. It's a bureaucratic thing, and it won't be noticed by many people. A few more of these, and all of a sudden you're talking about real change.