This is more like owner of a noticeboard being liable of things posted there.
Either have a human verify everything nailed (the upload filter) or just remove the noticeboard. Which one will happen?
Libaries and bookstores are more comparable to online shops. I have no qualms for Steam being liable for selling a pirated game. Shops should have verification on products being sold.
Abuse by Google is bad, but this is throwing the baby out with the bath water.