Yeah I know plenty of instances limit them making follows approval only. I also saw this line on a person's profile: "if you follow me from mastodon.social I won't approve your requests, migrate to a different instance".

I think the main reason is because mastodon.social has a lot of spam (or has had it in the past) though I'm sure that the threads issue and the fact that they want to federate and encourage federation with threads probably isn't going to help.