No idea....but here goes:
I can see no reason why heating by microwave or any other form of heating should be unsuitable for diabetics.
However, peripheral neuropathy, not uncommon in diabetics could make an excessively heated warmer (by any method of heating) undetected by the wearer with the sequelae of possibly skin burns and ulcer formation.
carlton....I have made all that up, based on a certain amount of medical principles, but perhaps another poster may have a better explanation.