there seems to be no objective reason for any abstract restrictions at all.
I always assumed that a hardpoint was more than an area of the hull; it was a load-bearing point that also had a power conduit (though depending on the PP, not all conduits are powered at the same time). So a turret fits on a hardpoint. Fine. But also a bay is much larger than a turret. Therefore it 'consumes' 5 or 10 hardpoints. And a spinal point 'consumes' many hardpoints ... eg. the PAWS-N 'consumes' 30 hardpoints.
(Also, since a spine weapon is essentially a linear accelerator, if (for example) a PAWS-N were to have 'x' of its hardpoints damaged the weapon as a whole would only be x/30 as effective. Of course the 'barrel' isn't the only part of a spine weapon, there's also the initiator, but that's interior and thus doesn't need hardpoints.)