In any case, I don't really think that an instance subscribing to a *specific* relay, even one with open registrations, constitutes opt-in consent for all its posts to be relayed just anywhere, by anyone, for any purpose. If my posts ended up on Bluesky just 'cause my Mastodon server relays to other Mastodon servers, I'd be upset.
This thing with tags.pub may be less bad than that, but Evan Prodromou 🇨🇦🇺🇸🇬🇷🇵🇸 (npub1764…nuj0) doesn't see an issue at all...