You do know there’s a big difference between a “default” option and a “mandatory” setting, right? Specifically that you do, in fact, have a choice to change a default?
Not forcing the user to proactively make a choice is not the same thing as denying the user the ability to choose.
Only insofar as some instances block communication from some other instances. Not mine though, that’s actually one of the reasons I picked it. That and it being by an org that’s older than the web and runs a public unix server and a bunch of retrocomputing type services as well as fediverse stuff. They started out as a dialup anime BBS.