We didn't have a paywalled internet before ads became commonplace. Most content on the internet is still user-generated and almost all of those users do not get paid anything from the ads put on their works. Hosting is quite cheap unless you want to run a centralized service that serves literally everyone.

If anything is a social contract then it's that if you want to provide a paid service you are up front about requiring payment. Ad-supported websites don't much care about that social contract because they think it's more profitable to pretend to be free when they are not.