Quote - Typical authoritarian thinking.

No. Authoritarianism is forcing someone to do something with their own property... Like a forum-goer dictating site policy to the owner(s).

Quote - But members of poser forums act like they are just tolerated guests, eternally gratefull and ready to be pushed around anytime like unruly schoolchildren.

They are tolerated guests. Administration can ban them, w/ or w/o reason, anytime they want to. They don't have to provide an explanation, they don't have to answer any requests for reinstatement in any way, they don't account to a social committee or a court or an official for why they banned someone (what a grotesque thing that would be, huh?)
FLICKR could have told everyone that didn't like it to get out. They didn't do that because it is not a good business decision, not because anyone else had any real say in the matter. Strictly a FLICKR administration decision, based on feedback. The way any business (at least successful ones) operates.

Quote - Sorry, but to me a forum is not a "private propety".

It's private property in every sense of the word. Someone owns it, they pay the bills to make it possible. If there is legal problems, it is the owner that takes the flak, and pays the fines. When the bills are due to keep the doors open, it is the owner that gets the tab. When profits come in (if there are any), the owner gets those too. They have 100% unrestricted power to say how their forum will operate.
In what real sense is it not private property?

Quote - The owner deserves a certain amount of gratefullness for providing the place, but his rights are limited as long as it is open to the public.
Especially if you using a forum to make $$$.

Limited by whom?
Do we really need (or want) some committee somewhere dictating internet website policies?
The admin. has 100% control of their own forum. The way it should be.
If I invite guests to my home, do I lose all right to tell them not to kick the walls in, or not to take a piss on the carpet?

Quote - ...make the real world such a miserable place to live in for anyone but the super-rich.

The real world is not miserable for me (And I am not super rich). The real world is not miserable for a lot of people; even a lot of poor people enjoy life. Shocking!
Besides, what other choice do you have?...

Friends don't let friends use booleans.