The problem is that Mastodon puts a second filter bubble on top of the old who-do-I-follow filter bubble. Except this additional filter bubble isn't controlled by yourself.
There is always a second filter bubble. In the case of non-federated sites, that filter is in the form of the policy of the site and also the algorithms used to sort feeds and replies.
They have a choice? Is it transparent for average users what is withheld from them and where? And where their own posts are withheld? If no, they don't really have a choice. If such a choice even exists.
Twitter is fairly lightly moderated I think, and the "following" tab gives a raw timeline.
With federated solutions, users have a choice.