I ran a website called the Motley Fool which has very active boards and was involved in the early days of establishing liability in these kinds of situations. Unfortunately, it is very clear that in the UK (not the US) the website can be held liable for defamatory statements posted on its boards. In practice, a website like pprune cannot be expected to patrol its boards incessantly and pull all posts which could possibly be considered defamatory. However, if a complaint is made, along with a request that a post or thread be pulled because it is defamatory, and the post or thread is not pulled within a reasonable period and it does turn out to be defamatory, then the website will be liable.
So, everyone can whinge as much as they like about 'sell-outs' and 'manipulation' etc, but it is a very simple case of the law of the land and survival. Don't like the law? Campaign to get it changed, but don't impute anything to pprune. They are just doing what they have to do.
QDM