I assessed a kid once for learning difficulties. His full scale IQ (an outdated, almost meaningless measure but fine for illustrative purposes) was off the chart, and yet his written language was appalling and his spelling was at a level about 10 years below his age. Ask him to write and it was almost gibberish, ask him to answer verbally and it was unbelievable just how bright he was. One of my brothers builds truck trailers and is almost as bad in written language, yet can eyeball a trailer and tell you where it will fail and by how much and where it needs to be reinforced to prevent failure.
I think it is wrong to presume that errors are from laziness. How a person is approached regarding their mistakes will make a big difference to whether they even come back, and to be honest most people who struggle to spell probably already know that they struggle, and don't need a faceless person on the internet to tell them. If it was that simple, I suspect their problems would have been solved by now. My sympathy is lower to those who are representing a business and have mistakes in advertising - no excuses for a business not to appear professional. For individuals in social settings, as long as they can get their point across, I think that is enough.