As Jim points out, G.M. could have sought a "design patent" for the Hummer H2. (For all I know, it did.) The drawback is that a violation would have to be close to an exact copy; according to this FAQ from a patent law office, a design patent "protects the ornamental appearance of the object exactly as shown in the drawings." The requirements for a trademark claim are not so demanding. (Trademarks also can last forever, while a design patent has a term of 14 years.) Hence G.M. is trying to get design protection it would not otherwise enjoy by pretending that its real concern is consumer confusion.