people make typos sometime without actually being ignorant. Heck, I even find I wrote there instead of their once in a while when I wrote something fast. I make other mistakes too, sometimes putting in a completely wrong word instead of what I had thought in my brain. Who knows why such things happen, but that is the main reason (if for no other) that there is value in checking what you wrote before you send it.
So, people that make such mistakes are not being careful, not thinking about the reader (that is the charitable view, that it is done by accident and lack of care for others, and I still think it is generally bad because it shows a type of selfishness). Many people simply are not even aware that the two words mean different things. It is difficult to be serious with someone when they are profoundly ignorant. You can call it grammar nazi if you wish, but the people making the mistake are the ones at fault, not the people who do the complaining or mock. Don't blame others for your own mistakes.