My guess is they're easy to steal, and would cause a money draining grey market. If they're all illegal, it's easy to stop, rather than checking the pedigree of every case found on the block.
The heaters are another possibility, but with all the dangerous chemicals available to consumers, I'm not sure why they'd be a problem. AFAIK, the heaters aren't especially dangerous.
This is the reason as far as I know. You can buy cases of the heaters (just not with the MREs).