I think the issue here is that it doesn't really matter what SAHMs say. Just their very existence is aggravating and triggering for some people on here (clearly) because, even on an unconscious level, it essentially triggers the question, "What does she think she's doing with her kids that I'm not?" This is the crux of it really.
The unavoidable fact is, that where there is the choice to be a SAHM (there isn't for everyone), there is no getting away from the fact that those families have taken that decision because they think it's better for their kids. This is the whole point of having a SAHP. Why else would they be doing it?
It's similar to someone whose financial situation affords other choices - such as the choice to use private schools. In a sense, you've voted with your feet there. It's obvious you've chosen the independent schools route because you can and you think it's the better option than the alternative.
This is why even a seemingly innocuous comment such as, "I wanted to be the one there for my kids" (which is, of course, true for a SAHM and the whole point basically) incites such a backlash - "I do all that and a job!" "You are controlling!" "Do you think I'm a bad parent then?" And all kinds of convoluted projections about the husbands of SAHMs and what they are like; or statements about their financial vulnerability. All mums are super-sensitive where their kids are concerned. Projecting is a way of feeling better about yourself - "eg. Well I have to work, but oh, all their husbands will all have affairs and they'll all be destitute ... yes this HAS to be true...."
Most families in the U.K. today are struggling financially or are broadly in the category of the "squeezed middle". These families need two incomes. Also, a lot of people in here will earn more than their husbands, so no wonder they can't comprehend being a SAHM. It doesn't matter whether they want to or not - it's simply not a choice most women can make anymore. Many women on here are in marriages with separate finances. No wonder they can't begin to comprehend how other husbands are happy to financially support a SAHM and think nothing if it. How can they grasp this? The most vociferous SAHM critics in here are probably with husbands who can't or wouldn't financially support them even if they wanted to be a SAHM. No wonder SAHMs provoke suspicion and defensive vitriol.
This is why these threads always run to 1000 posts.
In short, the truth is, yes, SAHM are SAHMs because they think it's better for their children. There's no getting away from that.
BUT - it does not follow that, because of this, they think they are better parents than women who work. They understand that not all women feel the way they do, or even have the choice. What other families do or don't do is irrelevant. There are as many ways to be a good parent as there are women in the world.