I think it basically comes down to the owner of the copyright.
Years ago, a guy found some stormtrooper armor in the dumpster behind a movie studio. He made armor based on it and sold it. Not as a 'on-the-side' thing, but as a full time job. Lucasfilm sent a cease and desist, but he didn't and they sued him. From what I understand, there were two problems. First, he sold it as "Stormtrooper Armor" with the emphasis that it was made from screen used armor. Had he called it something else, like "white space armor" he may not have had a problem. Second, he made this his sole income. If he would have kept his job and just made the stuff on the weekend he may have gotten away with it.
BUT - LFL (LucasFilmLimited) is pretty aggressive when pursuing copyright infringement.
In regards to Westerfield, they may have some agreement with MS/Bungie. MS/Bungie may not pursue copyrights as aggressively. There are many factors, but what it comes down to is if the copyright holder decides to press the issue, that's when the problems occur.
One difference between Star Wars and HALO - Actual costumes and props were made for Star Wars, so theoretically one could take a costume, say Boba Fett, make copies and sell it. HALO, on the other hand, is a video game and no actual props or costumes were made. So copying an existing set of armor is next to impossible. It's a different medium, and that could pose problems if MS/Bungie were looking to file a lawsuit. But again, it comes down to what copyright holder and how they want to pursue any potential infringements.
