Unfortunately the service does cost money to run, and it is hard to pay operating costs without some sort of income, so it's hard to fault the service for charging money for a valuable service, especially on HN.

This lawsuit (if it goes to court - many cases do not) would decide whether it is, or is not, "allowed". Still, contracts can contain all sorts of clauses, not all of them will be permitted/enforceable by law. Famously, non-compete agreements aren't enforceable in California but that doesn't stop them from appearing in California employment contracts.