Something similar has happened to me twice (different scenarios), and the only commiseration I can offer is that they are life's learning experiences. Now you know a little more to get everything mentioned down on paper before you sign. I find it hard to believe that the referral bonus wasn't mentioned in an email or other notification-level policy statement, but aside from that you're probably just going to have to eat it.

They certainly did weasel out of it, though. Obviously they were looking for ways not to pay it rather than to avoid bad blood. That's fine, karma, they stepped on you on the way up the ladder, etc. Not a lot you can do about it, probably (but I'll be watching the thread ;).