If I can adapt (and perhaps torture) your analogy, I'd say it's like Facebook currently has doctors who save lives and command high salaries, and janitors who change bedpans but don't have access to hot water and latex gloves. So inevitably, the janitors catch a disease (hospitals are filthy, after all! This is foreseeable!) and are no longer able to work, at which point they are replaced.
Given that the hospital can afford to pay the doctors, we might ask if they could splash out for latex gloves and soap for the janitors, too.
Content moderation job is not for everyone.
People are essentially asking if FB can increase salary/ perks by x amount because internet commenters are somehow not feeling good about current scenario.
Yeah, this is basically what I'm saying. :) Labour codes are developed over time. In the beginning, it's the wild west. Want to be a janitor and not use any personal protective equipment? Go right ahead! After workers keep getting sick, the government begins to legislate requirements around health and safety.
Awareness of mental illness is a relatively new thing. We'll probably see some developments in this area if the big tech companies continue to outsource moderation at scale. https://www.theguardian.com/news/2017/may/25/facebook-modera... is a nice article that describes accommodations that other companies provide to moderators. They include, as an example, monthly psychologist visits which continue after the person stops working for the organization. They also include training for the person's family and social support groups.
It would be like expecting doctors to clean bedpans. Now it is a neccessary task and those who do so should receive appropriate respect even if it is just "Thanks glad I don't have to do it." but anyone off the street and willing to do the dirty job could do it without a doctor's training which they spent over half a decade on to be /entry level/. Plus incredibly inefficient for effectively paying say $100/hr to a janitor when they could be saving lives instead.
Now asking them to do it in a neccessary and justifiable situation (say posting in an aid camp cut off by weather or in space thus making sending any extra bodies expensive) is one thing and they would be in the wrong then for refusing out of pride.
Absent that neccessity it shows poor sense of their actual value until medical degrees and skills become just as common as the skills to clean bedpans.