I'm not sure why BD139 left outside of what they posted here.
Ultimately, BD139 and a handful of others left TEA (and the entire forum) because over "over-moderation" of that thread based on users reports. They didn't like posts being moderated in a thread I guess they considered their own little community space.
They were offered their own private space with their own appointed moderator, and still they didn't take up the offer though, they went and formed their own TEA forum.
Some returned, some didn't.
I would actually be more interested in the root cause of technically contributing members being banned.
Usually it's because they refuse to stop doing something when given multiple warning.
Sometimes it's because of a personal interaction and attack of a moderator. You won't see any of that stuff.
I've had threats against me and my family. The forum has been attacked with scripts, had CP post on it, had the server shut down for false claims, etc. Some people just get really nasty when you try and tell them please don't continue to do something.
Searching their history, they often appear to get caught up in one of these off-topic weaponized mod discussions. I suspect they post something that was bad enough for them to be permanently banned, and their post removed. Rare I see anything happen to the people who start these threads.
Why should the OP get pubished for starting a thread that then turns sour because of other users? "Incitement"? "Trolling"? The forum would die of over-moderation in short order.
Remember how BM139 and other top contributors left because of over-moderation?
If you create a thread similar TEA where anything goes, no one would be banned from posting their opinion, mods would not intervene unless there was illegal activity, I would support it.
You are in the minority. We have had polls on doing this multiple times over the years and the comunity response is always a majority, don't allow it.