Your entire comment reeks of "we shouldn't fight fire because that puts firefighters at risk".
There are no 100% ethical solutions to every problem, real life is a compromise. You can get better ethical results by allowing those workers to get adequate monetary compensation for their work and seek medical help if they need it. Otherwise what's the solution, allow everyone to read the same stuff? Why is that more ethical? Is it more ethical for the random user (who may also be a suggestible kid, or a person belonging to a persecuted minority) who reads it? Is it more ethical for the developers who get their game review bombed by fascists and bigots, and see their source of revenue diminish or fizzle out because of it?
As for the legal responsibility, it becomes so when the platform is complicit with the users writing hateful stuff. You are not responsible for the random shithead declaring his love for Mein Kampf. You are responsible for the hundreds of users who do, while you repeatedly ignore the reports of their misconduct, thus implicitly accepting and normalizing their behavior.
Additionally, when hateful behavior is accepted and normalize, human shit stains will come in drove and multiply the problem tenfold. By moderating their spaces, they would prevent a lot of those hateful messages from being written in the first place.