Its actually a large-ish walk-in cooler in Bell services, the same place they'll hold your GG/WGS orders if you aren't in your room when they arrive. I would suspect the area is under video surveillance, and I doubt any Bell Services or Front Desk CM would be willing to risk their job by tampering with your food just like they wouldn't risk their job going through your luggage that you have held at Bell Services or delivered by DME.
Yes, if you wanted to retrieve milk for cereal in the morning, you would have to go to the Bell Services counter outside the main building, have them bring you your milk, use it, and have them put it back for you if you didn't use the whole container. The point of the $15 per night room credit is to offset the cost of you going to the pre-pack section in Everything Pop and getting a single-serve milk, snack, or other beverage that you would have otherwise bought in advance and stored in the fridge you expected to be there. Of course you could just apply that credit to souvenirs and food throughout your trip and use the money saved to buy a fridge (an 8-night stay's worth of credits would buy a 4.6 cubic foot fridge/freezer from walmart.com including tax with free shipping) then just sell or even give away the fridge if you don't want to ship/take it home.