Doubt YC actually charges anybody for use of the board. More likely they're offering a useful service free of charge to collect something even more valuable: at

Doubt YC actually charges anybody for use of the board. More likely they're offering a useful service free of charge to collect something even more valuable: attention. YC gets free advertising that passively selects the most relevant users, and users get a free link aggregator with unobtrusive ads relevant to our interests. Everyone wins!