I think it really comes down to two things:
1. It is (generally) obvious if someone has a speech/language disability when you interact with them face to face.
2. Sheer probability dictates that it would be ridiculous to ignore a metric (written communication skills) that I find useful in evaluating people for fear of offending a VAST minority of cases. What percent of the time do you think that someone exhibiting incorrect spelling or grammar online is doing so because of dyslexia or another disability, as opposed to simply being lazy or unprofessional?
I would never knowingly ridicule someone who suffered from dyslexia or a similar disability, but I think we have to be careful of becoming so politically correct that we are afraid to criticize or hold anyone accountable for anything.
It's pretty prevalent 1 in 10 people (at the top end) that you interact with has dyslexia of some form. Among people in the arts it is much much higher, I am making an assumption here, but I would assume it is significantly higher on HN given that their is a population of designers that frequent the site. It is far more prevalent that speech related disabilities, but less recognized because people are embarrasses about having it, some of that embarrassment comes from the fact that unlike a speech condition it is acceptable to highlight their disability, many times to discredit their argument.
as opposed to simply being lazy or unprofessional?
I can't begin to help you understand how many times I have been called lazy for not being able to spell, and how frustrating that is. That is the problem, you assume the majority are lazy people that cannot spell and don't want to learn, so you immediately assume someone is in the majority, because well you set up the odds that they are. But if 1 in 10 suffer from it, and the prevalence of people on HN exhibit spelling mistakes at close to the same rate would in not be just as valid to assume that maybe those that do exhibit them, may be in that 1 in 10 population.
I would never knowingly ridicule someone who suffered from dyslexia or a similar disability, but I think we have to be careful of becoming so politically correct that we are afraid to criticize or hold anyone accountable for anything.