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.
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.