"Usually I just lurk because I don't have a whole lot to say in such lofty ccompany."
There are many different ways of contributing to the success of the forum if you were inclined to make the effort.
"Lately though I have noticed several instances of censorship by the moderators of this forum."
That is simply not true. A number of posts were edited/deleted some weeks ago during the crackdown on people freeloading at PPRuNe's expense to promote their own business ventures. With only one exception, contributors to this forum accepted the decisions with good grace having had the reasons explained to them. You are the exception.
There is virtually no 'censorship' on Rotorheads because, with thankfully very few exceptions, our contributors habitually behave in a mature and sensible way. On Rotorheads, 'censorship' is necessary extremely rarely, and action is taken only after consideration of the potential legal ramifications by a lawyer.
"Correct me if I'm wrong, but at least in the U.S. etc"
What you say may be correct under US law. It is not the position under English law. PPRuNe is based in England.
"I would want to rely on the fact that opinions of contributors to a public forum are not endorsed by the forum's host, as a first "defense".
That is not a defence in English law.
"I don't think that a pilot's forum especially should be pro-actively protecting manufacturers from contrarian opinions."
Nor do I, and we do not. (See explanation above.)
"If Robinson, Bell et al haven't complained so far, I believe they never will."
So far as I know, Robinson have never complained. On the contrary, some time ago Frank Robinson personally and patiently took a great deal of trouble to answer in detail the pet theories of our most actively anti-Robinson contributor, despite the fact that some of the accusations were against him personally. Some manufacturers have made valid and reasonable complaints in the past; I am not prepared to disclose which. What I can reveal is that all the major helicopter manufacturers now follow Rotorheads closely.
As for your belief that because something has never gone wrong so far it never will, I hope for your sake (and the sake of your passengers) that you don't adopt the same approach to your flying.
"It would seem to me .................... etc"
It may well seem that way to you, but we do not have unfettered freedom of speech in the UK. The laws of libel are but one example of the restrictions which apply.
"As a professional pilot, I welcome the whistle blowers of the world precisely because they ask provocative questions ..... etc."
So do I. But we also have a responsibility to act responsibly. That includes guarding against the forums being used (or abused) by disgruntled individuals and 'cranks' who have their own agenda, and those who inadvertently 'cross the line' in their enthusiasm.
"For me it is literally a matter of life and death to have potential problems brought to light in this way."
I agree, but it depends what you mean by "in this way. If you read the discussions more carefully, you'll see that many problems are brought to light and discussed.
"For this reason I would much prefer if the moderators were to refrain from any censorship until .......... "
Your preference has been noted. Our preference is to act upon professional legal opinion in order to protect PPRuNe's interests and ensure that it continues to provide a valuable and free service to the aviation community.
Edit
Just thought I should add that if I was wearing my 'contributor' hat, and didn't have any responsibity to protect PPRuNe's position, I too would be arguing for carte blanche unfettered freedom of speech! 
But, if you agree to do a job ..... etc.