If you'd grabbed "celtics.com" do you think you would have a legal right to it? What about McDonalds.com? Why is this different than general brand trademarks as they relate to domain names?
It seems to be a conflict if you're in the same "domain". i.e. If I grab @celtics and talk about Celtic language and culture, it shouldn't be a conflict. If I grab it to build a following about the team, I'm piggybacking on the effort/expense of them building that brand and deserve to have it yanked.
IMO, it's very similar to the Nissan.com case - the owner of that domain is named Nissan and has a business with that name. He got there first, and he's got rights. However, if it was some random guy who put up a splash page or a nissan car site, it's trademark infringement, no?