I don't think this is true. I see "Boosted" posts in my feed from people I don't follow. Maybe this is a server specific feature? I am using Hachyderm.io.
This is a standard Mastodon feature. You will see post that people you follow "boost", but not things they only "favorite".
My understanding is that the Mastodon devs are trying to separate "increase the distribution of a post" from "give positive feedback to the post author".
yeah people can boost posts on mastodon (essentially a retweet), but you don't see the asinine algorithmic things like replies to people you don't follow (unless you go look for them)
I do like that there's no reply component to boosts though. You boost it up or not. No sniping.
