Machines are better at solving captchas than humans. No, really.

It works because it is different from other captchas that all have ways to workaround already automated. Having a niche problem for a niche community works surprisingly well for weeding out the widely cast bots that are designed to solve the common captchas. Though it wont do much for a dedicated attack - I don't think they have had much of a problem with these and the mods can always change tactic when it no longer becomes effective.