Actually I don't think they are made to feel unwelcome purely because of spelling and grammar sloppiness, although it does rankle. I think it's more about what they say, rather than how clumsily they say it. As someone said on another thread yesterday, there is a culture of wanting to preserve the character of the site, and that causes some people to guard it jealously and put up barriers to keep certain people out.
I also think that some people come onto MN and it changes them; they discover a whole new elite world that they want to be part of. They become more aspirational, more aware of what and what is not considered socially acceptable among the middle classes and the educated, they become more discerning about things like food, or education, they may make an effort to modify their speech etc, as the MN effect starts to rub off on them. And they sometimes need to show they 'get it' at every turn; prove they are 'one of us' which why we get so many hackneyed, cliched posts about 'Oh, haha, my DS is so middle class - he asked me for olives in the middle of Waitrose the other day! How embarrassing!' said with utter glee. I think they can be the ones who are hardest on the poor unsuspecting pariahs who stumble onto here - they want to earn their MN Intelligensia stripes, and they think that's the way to do it.
I think there is a subconscious need to dissuade people from sticking around if we collectively sense that the fit is not right. Like when Julia Roberts in Pretty Woman goes into the posh boutique and the ladies look her up and down and say 'I'm sorry dear, we don't have anything suitable for you in here.' 
Let's face it, while it's pretty unpleasant to witness it at times if it didn't happen and we were equally welcoming to all, we'd soon be just like netmums and 80% of us would no longer want to be here. The conversation would bore us and appall us in equal measure. Netmums has its place; it serves its demographic well, but we are not (by and large) its core/target demographic and I doubt they want us lot on there either, quite honestly. Well, the owners of the site might, as we probably have more disposable income (on average) which is good for bringing in advertising revenue, but I doubt we'd be very welcome on the boards, with our po-faced standards and our use of big words, and what with us thinking we are better than the little people. And we do - let's face it otherwise we'd be over there. No point denying it.
It's like when people go for years to a particular favourite holiday spot and then getting all huffy and indignant when it is discovered by the masses in football shirts who lower the tone. We are the site that has a sign outside saying 'No baseball caps allowed'. But we all know it's not because of the baseball caps themselves - it's because of the stereotypical behaviour of the people who wear them. We all secretly know that if David Milliband walked into MN in a baseball cap, we'd turn a blind eye, because we'd trust him not to put a chair over our head if we looked at his bird. 
There are sound reasons for not allowing text speak on MN, but you've got to admit it's also handy for weeding out the undesirables, whilst conveniently taking the moral high ground about dyslexia and people who are forrin.
I am not condoning any of the above BTW, I just think there is an unavoidable element of human nature about it. I do not approve when it gets too obvious and too nasty though. Especially if the poster sounds vulnerable and unhappy. But apart from a handful of people who have a bully mentality I honestly think that rarely happens.