Well, there's a 2 part answer to that. MARPAT, which you are referencing, contains little EGAs (Eagle, Globe, Anchor - the trademarked logo of the marines) and it's intent was to both distinguish it from other branches uniforms and reduce the amount of commercial knock offs.
For rank, we've borrowed rank insignia from predecessors, so you can't really trademark that.
The rank honestly means nothing to anyone outside the military. And it only matters when you're trying to figure out who has the authority to make certain decisions. Faking rank or impersonating a different rank will get you into trouble FAST.
Unlike currency, which has value on it's own, the rank means nothing without the paperwork to back it. There's paperwork behind your rank, and THAT'S what matters. Not the insignia itself.