Microwave explosions aside, the actual reason that you should not microwave a frozen thawed feeder is because the microwaves will slightly cook the meat on the rodent. Your ball python is not designed to break down cooked flesh. I know that impaction is one of the problems that your snake could suffer and I've heard that the cooked portion of the rodent can actually rot inside your snake, but I'm not sure how true that is.

Hope this helps.

-adam