I imagine it's the same as pool hopping and using refillable mugs on subsequent visits. It's probably not something that's enforced 100% of the time. Following some of the fridge swap threads though, a number of them do go "missing" and you have to wonder if it's because a Disney employee confiscates it. I thought coffee pots were supplied by housekeeping if they weren't already in the rooms and although inconvenient for some, toasters and microwaves are available for guests to use in the food court. Unfortunately, it's like anything else, Disney could limit it to certain small appliances, but there are always those guests who push the envelope by showing up with crock pots, hot plates, etc.
I would definitely suggest dropping an e-mail to guest services though if you are at all concerned about what can and cannot be used in the room
wdw.guest.communications@disneyworld.com . They are usually very good about getting right back to me whenever I have a question and it's best to go directly to the source.