> Why makes this such a difference?
If it has dedicated silicon for decoding MPEG, MAME would have to emulate the MPEG decoding functionality, without a patent license to do so.
If the game has a general purpose CPU or DSP running an MPEG decoding program, MAME only needs to emulate the CPU or DSP. MAME won't contain code requiring an MPEG patent license. Sure, you still won't have a license to run the program, but that's the case with most ROMs anyway.
Bunch of ROM pirating MAME users all of you