I think you should re-evaluate that assumption:
https://www.google.com/search?q=mastodon+instances+blocks+ga...
https://www.google.com/search?q=mastodon+blocklists
https://www.google.com/search?q=mastodon+blacklists
https://www.reddit.com/r/Mastodon/comments/kw8jht/how_do_i_s...
Also, expand the ">Moderated servers" heading and scroll down through the instances they block:
https://mastodon.social/about#unavailable-content
Why do different Mastodon instances decide on not federating some of the other instances?!? It's because the owners pay money for running their particular server which makes them feel entitled to run it the way they want.
The "pick any server, it doesn't make any difference, you'll see _all_ messages anyway" -- is not realistic given that each fediverse node administrator can exercise their freedom to choose what messages their server accepts.