This stuck in my mind but after a good sleep ...
CMG and BMG with a few exceptions are mounted in some kind of frames. The Panther D, an exception, just had an openable vertical letterbox flap through which the BMG was poked. The JgPz IV had one or two proper ball mounting(s) covered by a rotating cone, but the BMG is very rarely photographed fitted. So in general they took at least a little time to remove.
With the StuG/StuH III G (most production) and H35 the AAMG had a mounting point that could take the AAMG but the manning crewman would have to be in an awkward and utterly exposed position and pose to fire other than through the VCA. In the case of the StuG a pin in the small U shaped mount attached to the MG dropped into one of two sockets welded to the AAMG shield (one below the MG hold, one on the top edge). It would only take a second or two to mount/dismount the AAMG. There were no fiddly screws/wing nuts/levers to open, just lift and remove. As far as I know the H35 was similar. Both MGs were nothing like the weight of the equivalent US 0.50", either.
So restricting fire to VCA would reflect normal firing practice, with one's body at least partly shielded by armour. In a emergency (eg CC), the MG could be very, very quickly dismounted and fired in any direction. So though an annoying inconsistency in the rules, it could be said to have some basis in reality.
So allowing its full use in CC is not as bad as it seems.
For those who like special rule tweaks:
The StuG was one of the few AFV where the AAMG was manned by the loader, usually manned by the commander in other AFV. Reduce the RoF to 0 if the AAMG is fired?
The KV1 turret had 3 crew, commander/gunner, loader and the turret rear MG gunner who also manned the AAMG or could help load. Allow [1] RoF OR fire RCMG OR fire AAMG in addition to normal fire?