As far as I'm aware, there isn't such a policy, although you'd need to get confirmation from the British moderators.
Having said that, I think it's important to look at context when making claims about moderator intervention. You only have to read back through a thread like this or the Sarah Duchess of York thread to know that there are plenty of negative comments and plenty of questions where the royals are concerned, and they haven't been deleted.
As a rule, negative comments and questions about forum moderation should be dealt with in the Support and Feedback forum or via PM so they don't disrupt the flow of conversation about the thread topic. If we're talking about negative comments and questions about the royals, I'm not seeing evidence of systemic censorship.
However, especially in the York threads and the threads about William and Kate, there's increasingly been a very unpleasant tone. I know the British mods have been getting complaints about these threads from members and other mods (apparently a lot of people don't seem to realise I'm no longer a British mod so I've been cc'd on a few of them). There may well not be a clear demarcation between thoughtful criticism and outright bashing in any given post, but it isn't all that hard to tell when a whole thread has slipped across the line. And the Sarah thread slipped across that line quite some time ago. The mods have been trying to nudge it back in a low-key fashion, but several people were too engrossed in "Sarah can't do a thing right and anyway she got her toes sucked so let's poke spiteful fun at her" to notice, hence the firm intervention of a couple of weeks ago.
I understand that in this thread there's a different issue, because there do seem to be serious and fully legitimate questions about abuse of privilege and coverups and so on. However, the point here is that some people are saying that this is just how it is with royals and others are claiming that it's specific to William and are using that opinion as a springboard to "William is a pathetic disgrace" comments which seem to be beginning to dominate the thread, to the detriment of the discussion itself. I think the issue of whether the current abuse of privilege is ongoing or is unique to William may be a case where people will just have to agree to disagree, but I must say I can't see any evidence where posts advancing the latter opinon have been removed.
The moderation team across the whole forum (not just the British threads) is trying to avoid having threads turn into platforms to unleash venom about controversial royals, ex-royals, and associates of royals. There are other forums on royalty where spite is consistently passed off as wit. Most outsiders reading those threads can tell that this is a transparent excuse for vicious piling on, and the atmosphere in those forums has a lot in common with playground bullying. That isn't the sort of atmosphere we're striving for here.
The above are all general points and not directed to your posts, but I'm going to make a personal observation here which of course you can feel free to disagree with. I think that if someone had been talking about Camilla in the same tones that certain people have been using to talk about Sarah, you wouldn't be so fast to refer to them as "interesting" posters who it would be a shame to lose from the conversations. It's very easy, and I know it's just human nature and I'm as guilty as anyone, to see things through the filter of one's personal preferences. However, the moderators do have to try and provide as level a playing field as possible, while of course acknowledging that when certain royals behave like idiots, it's perfectly appropriate for people to call them on it. Up to a point, of course.
With regard to nasty posts about Camilla would be unacceptable, whereas comments about Sarah are, it happens on a regular basis and appear to be ignored. However I accepted some time ago, a style that one person likes would be revolting to another, even the comments about a 61 year old having wrinkles are laughable when you imagine how some 30 - 50 year olds might look, (and I most certainly do not mean you).
With regard to the posters who have apparently left, we did not all agree on every royal, but some of the information and discussions were thought provoking. 
I perhaps got a little carried away, a 
moment!
