why is odst called "halo 3" odst
It used the Halo 3 game engine, instead of a new or tweaked version, like the others. (Even Firefight was done using stock parts of the engine.)
No, I don't think it's cheating. That's like saying it's cheating to read Star Wars: Millennium Falcon, because Luke has 4 lines in the entire book, and doesn't actually appear in it. I think it's using a different sense of perspective to tell a part of the story we wouldn't know otherwise.
The Prequel Trilogy tried too hard to focus on Vader, while completely ignoring the background of all the other characters. Padmé just kinda shows up, and no one knows anything about her until the end of the first movie, she's not developed past a disjointed version of her original character in 2, and in 3, she simply serves as the catalyst for Vader's fall, and only coincidentally gives birth to Luke and Leia. Meanwhile, Anakin gets all the attention, and the story, if you can even call it that anymore, focuses around him. It's like Halo 3, where Johnson and Miranda didn't mean anything to you. If it weren't for the major side characters, (Johnson, Miranda, Arbiter, Cortana) I wouldn't have enjoyed it because all it does is show the Chief's final battle. Those characters tied the one-man-army down to the "real" people he knew and cared for. Arguably, the Master Chief has no character development, whatsoever, except that we know he wants his hologram girlfriend back. We don't even see anyone else develop character. In ODST, while the rookie has no development, we see the entire story through the eyes of his team, and we see their characters. In Reach, we see the entire team through the eyes of someone you can get a sense of emotion out of. Noble Six has human movements, and he hesitates when he speaks, sometimes, showing that he knows the human cost of everything that's happening. You only see Master Chief move from his normally-military-perfect stance once, when he slumps in the Forward Unto Dawn.
On a side note, I noticed Firefox's spell check recognizes Darth and Vader to be real words. Also, Obi-Wan.