Namely, that the doctor doesn't want to be 'spanked' by the pharmacy board for prescribing too much.

Nahh.. I can't say I blame him (them) - Doctors prescribing for OP's, I mean. I know where your original idea came from because I thought the same thing for a long time - that #90 was the 'magic number limit' - Mostly because most OP's set this as their upper limit with controlled meds. But I think that - although a doctor may be scrutinized harder for writing more than the basic '3 per day' since it is 'the norm' level, I don't think it's a hard-coded law. But I can't say I blame the doctors for being cautious with it. So many OP's do set #90 as their upper limit though - I dunno, I guess it is possible that it could be an individual 'state' law somewhere - maybe wherever the OP resides. Who knows. I know it is not a general rule though, since a few OP's, as well as of course local doctors, can prescribe far more pain meds than that per month.