I'll bet it's just a High Pass Filter that's programmed to limit bass after a certain volume.
The way I see it: If Mazda is smart enough to limit Torque in 1st and 2nd gears on MS3's, and they're smart enough to program a throttle plate adjustment after 5500 rpms to prevent damage to the engine, then theres probably a good chance they cock-blocked you from blasting your "Bose" system to it's potential and possibly blow the speakers.
In the end, it'll only save them money on Warranty replacements.
My SS has Bose and it sounded pretty weak with the OEM radio. When I swapped out with Aftermarket, the Bose components got WAY nicer and had some oomph! Maybe it's a Bose programming thing? Or better yet, maybe the Amp is overheating?