That's not a fair analogy though. To be fairer, you'd have to monitor said footpath 24/7 and have a robot and/or a number of people removing milk crate signs that you deemed inappropriate for your foothpath. They'd also move various milk crate signs in front of people as they walked and hide others.
If you were indeed monitoring the footpath for milk crate signs and moving them, yes you may be liable for showing or not removing one to someone it wouldn't be appropriate for.
That's a good point, and actually the heart of the issue, and what I missed.
In my analogy the stable sidewalk that can hold the milk crate is both the platform and the optimization algorithm. But to your point there's actually a lot more going on with the optimization than just building a place where any rando can market self-asphyxiation. It's about how they willfully targeted people with that content.
If you were indeed monitoring the footpath for milk crate signs and moving them, yes you may be liable for showing or not removing one to someone it wouldn't be appropriate for.