Hey yeah I saw this too! I thought it was only directed to one individual, but wow, there ya go.
Yeah it doesn't make much sense to me the whole you can only train with us if you're one of us thing... I've enquired once to a large Goju ryu organisation just in participating in an online session, and I got a similar response, only members.
Yet I've also encountered many groups that are so incredibly welcoming and one recently let me come to a private seminar they held with Hanshi Tino Ceberano.
I definitely prefer the open and sharing attitude, all part of the one family. The only reason I can think is that a non-member may take away the spot of a member and they'd miss out. So I understand that view I guess. But yeah, don't know.
Here's the most I could dig up: JTK appears to be a new association (I don't know exactly how new), that was founded by Masahiko Tanaka. He had been with JKA since the 1970s. Again, I don't know exactly how new JTK is, but it's new enough so that - as of today - Tanaka's Wikipedia page still shows that he's with JKA.
What this shows is that they parted ways on bad terms. If this was due to any wrongdoing on Tanaka's part, then I could understand completely.