It would be a tax because the existing members, who are not required to pay for the services (FR), would not have a say in whether the subscription was levied or not.
No, it would be a case of FR saying they are electing to run the site differently than in the past. The users don't own the site, the servers or engage in bandwidth contracts with service providers. The owners of the site do. Free membership means that someone else is paying for your user access.