you can eat or drink whatever you want whenever you want

Not quite. I don't have any candy easily available to me (I suppose I could buy more, but that would be a pain), aside from what's in the machine. Theoretically, I could eat that whenever I want, but I have some pretty strong incentives not to do so. I've precommitted to not taking any candy out, and I don't want to break the precommitment (plus doing so would probably ruin the system forever). And, of course, there's a very well-placed Schelling fence helping me stay honest, so it's not even that hard.