By "right to a name" I'm speaking specifically about Steve's right to own the Twitter username celtics. From a "first come, first serve" perspective, I suppose he has a right to use the username... until Twitter deems otherwise.
The terms of service said they could come and take it, and Steve's not paying them any money for exclusive use of the name, so from that standpoint I don't feel he has a right to use the username any more than the Boston Celtics do. It's Twitter - who owns the service - that I think has the right to do what they please.
I'm not saying that I agree with how they handled it and I can see why Steve is pissed off, but I don't think they have any obligation to let Steve keep the username.
I was kicking around a definition involving trademarks. In general, owning a name that can be seen to infringe a trademark is a problem, no matter how you slice it. The Celtics baseball team have an actual, legal claim to the word "Celtics", which is not unlimited by any means, but is certainly stronger than "some guy registered this name on this site".
Now, had he been some sort of Celtic organization (and I mean the ethnic group, not the team) it might be a more interesting discussion, legally.
(And I'm just skipping out on the ethical issues.)
(I've actually got some potential thoughts, but I'd like to not poison the well.)