Deciding to federate means accepting content from that other site. A mastodon instance federating with Threads would mean that Threads content would be visible on that instance.
However, if an instance decides to not federate with Threads, Threads can still get the data from the instance.
With the permisivity of the protocol, Meta (and anyone) can consume any ActivityPub data they want.
Compare this to email. A mail server can block incoming mail from some other mail server (e.g. a known spam farm). However, mail servers don't block people who have email accounts on their servers from sending mail to abuse@spammy.mail.server
Trying to change it so that federated content only goes out to pre-approved sites (because face it - if threads.meta.com is blocked it is would be trivial to spin up notthreads.notmeta.com and consume the same data) would fracture the fediverse and make small sites difficult to federate with... and lead to very much what we currently see with email - you can't stand up your own email server and send mail to gmail... which is very much what federation is supposed to not be about.