You don’t need a contract to sue someone in California. There are labor laws meant to cover situations that are inequitable or unfair. In my mind, having mods do all this work for the benefit of reddit (eg. Free labor) is unfair and seemingly rises to a level that should be investigated.
I replied to this type of comment in another thread. Reddit is governed by California law. There may be something here, since reddit was benefiting off of free labor. Volunteering might not actually matter, since moderators were putting in actual hours for the benefit of reddit, while not receiving anything back. Without mods, reddit wouldn’t have the power to moderate all the subs, thus, reddit relied on mods to do unpaid company work which would have cost reddit millions per year if they were to pay moderators. I think there’s a thin case here.