The problem with defining Join vs Follow is that in doing so you break or technically “outlaw” software which is currently using the Follow construct e.g. Friendica, Gup.pe (perhaps others). This is no longer about defining a specified behaviour but managing the transition of those projects that would be affected by the change. And in case you haven’t noticed, project developers in this space tend to resist authority.
I’ve outlined a possible solution to circles/aspects in Mastodon elsewhere. It involves binding a conversation to one of the two available addressing/delivery models using ‘replyTo’. Once bound, cooperating user agents would not providing addressing or privacy scope fields on followups if/when the conversational “top-down” delivery model was in effect (e.g. the recipients and privacy scope of the entire thread were pre-determined by the thread creator). I believe that is currently the only thing preventing that project and other microblog projects from playing nicely with conversational systems and also supporting groups/aspects. Playing nicely is a good thing. We support all of Mastodon’s non-standard privacy scopes. Is it too much to ask that they support ours, especially since ours is actually written into the ActivityPub spec? Granted, the spec left out a couple of things that are absolutely critical to making it work correctly.
I can only laugh that some would consider permissions/privacy and identity/data redundancy to be out of scope in a social network environment. I consider them to be the only things that really matter; and will continue to walk this path mostly alone it seems.