I am questioning the presupposition that one chooses to make any account at all.
Using your analogy:
I don't need to create an account to view a subreddit or reddit.
I don't need to create an account to view federated instances.
When I publish content on any social network I am always thinking about who it is intended for.
I don't post the same things on Facebook that I would on Instagram, for example.
Users have to make the same choice of what to publish with any other social media service.
Think Mastodon instances (hosts) as reddit subreddits.
Basically you are creating an account in not universe (reddit) but on instance (subreddit)
And this subreddit is deciding, which other subreddits you can see with your account. (fediverse)