I don't think mumsnet are saying the matter can't be discussed but I think you're being little unfair to Cameron.
Of course as a rich man with a disabled child his experience is different from that of someone without that money. Of course.
But he WILL have many, many shared experiences with other parents of disabled children beyond questions of money and funding.
I had a very difficult childhood with a violent alcoholic for a father and a violent, narcissist bully for a sister. But I'm also very middle class and relatively well educated so some of my experiences will be much easier than those who came from a more deprived background, but some will be just as hard, irrespective of background.
So, firstly, any experience of disability will have utterly transformed his knowledge of understanding of these issues and is much, much more that if he had not had that experience.
Secondly, it's entirely possible that as a Human being, who will be grieving for his son, that he's more than a little sensitive about this. To not allow this is to dehumanise him; is to suggest that his experiences can be wholly dismissed because they are not 'equal' to other people's (as he is very rich).
I'm not an apologist for David Cameron the politician, I have and never will vote Tory but the tone of some people is very unpleasant and smacks of the very cynicism that he is accused of.