Mastadon (and the Fediverse in general, to some extent) has problems with discoverability and the average user finds federation confusing. People tend to either use microblogging to see what’s going on with people they’re interested in or to broadcast their activities to a large group of people, and Mastadon currently doesn’t fit that niche very well.
I’m guessing that’s how they plan to get around that. They will leave the toggle enabled for people registered in EU countries, and disable it everywhere else. A fairly risky way to handle it in my opinion.