The thing about trademarks is if you don't defend them, you de-facto loose them. In other words, if people start using your trademark without your permission and you don't go after them for it, you've just lost your trademark.

It's still a slimy move, though. They could have given the guy a free license to use the trademark this once.

And, yes, a single letter stylized D can be a trademark. Pretty much *anything* can be a trademark as long as it's not already used as a trademark somewhere else. Movie ratings are trademarks, you can't say your movie is rated R unless approved by the MPAA.