This is a private house in the same way that a store is a private business.
Yes, both are open to the public, but only (with a few minor exceptions) for the purpose of the business. Even though the local fruit stand may admit any member of the public during business hours, it can prevent its visitors from selling their own wares in the store and can expel anyone who violates the store's rules of conduct or who presents a danger to the owners, employees, and other customers.
This is Mr. Petersen's house. He has invited the public in to read what they wish, and has invited those members of the public who are willing to follow certain rules to post messages here. He has chosen to have the messages and threads moderated. Whether we agree or disagree with his decision, it is his decision to make, and not ours.
I think the level of moderating will settle down as FT posters get a better understanding that the guidelines about posting will be enforced.
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"Yes, but at least mine will be found in a first class seat." -- Peattie and Taylor