I'm not sure the analogy really holds even though I agree that it is not reasonable to expect these people to perform this role full time.
Bullying, threats, the list goes on.
Many other jobs (e.g. in healthcare) are just as taxing and equally responsible for lives, but fail to enjoy these protections.
Ideally something like this should make it into a law or something similar, because trusting FB to do the right thing is probably not the way to go. (Having said that, that law would apply to only a handful of companies might be difficult to put in place; and then they'd outsource the job to "friendlier" country...)
I suspect the proper reason is because they might endanger others.
Outside of France, the law generally doesn't protect you too much from taxing yourself on the job.
Moderators should look for a different job, if they don't like it.