This is a very straightforward exchange - moderators spend time moderating in return for the endorphins and jollies this activity brings them. By 'leave' I mean 'stop moderating'. At any time, any moderator can stop moderating at the loss of nothing more than the endorphins and the gain of extra free time to devote to some other hobby. There is a glut of people who would take up their spot. For nothing. That's where the heavy competition is - lots and lots of people enjoy doing this sort of thing, for whatever reasons. It's not like there is a shortage of internet message boards and in most of them, calls for volunteer mod positions are flooded with applicants.
Reddit is not benefiting from the inexplicable altruism of some sort of rare and highly valuable specialists. And for all their talk, the mods know this too because they did not even threaten with a walk-out, let alone attempt one. Their only leverage was to force their users into this spat with the admins.