Anthropic and co basically have the opposite policy (for paid users): if you get sued for copyright infringement, they will indemnify you. That means they're confident it's not an issue.

By the way, do you have a source on Midjourney spitting out copyrighted stuff all the time? Does it happen at random or when users intentionally steer the prompt in that direction? I suspect it's the latter but I admit I'm not really familiar with this tool.