Spelling, punctuation and grammar matter at work, job applications, in published materials (books, restaurant menus, company websites) and on road signs etc.
They do not matter on chat forums on the internet, as long as the meaning is not changed.
And it is discriminatory against people with SNs or if English is not their first language.
If I posted that I thought that a child I saw in a supermarket was badly behaved and his parents should have controlled him better, there would be armies of posters going 'perhaps he had SNs, autism, asbergers, etc etc,etc'. They would say 'Perhaps the parents were having a stressy time and difficulty coping'. Don't judge a snapshot etc etc, and rightly so.
But if someone makes a simple, inconsequential little error in their posts, then loads of people just can't help pulling them up on it. 