I would also like to simply add a bit to this as well. I agree on all counts and have made most of them (if not all of them) at one point or another. But you bring up a good point that I never was really able to explain: the education a poster has received.
Depending on what the education (not necessarily the level of education) a certain person has received plays a large part in their "spelling abilities". If someone earns a degree in Mathematics or Engineering or one of these less "English-filled" degrees then they are at a disadvantage for spelling right off the bat. This is because they are not required to read as much or as diversely as an English or Philosophy degree. Therefore, they may run into words during daily speech and conversation with people, but may never SEE that word. To use the above poster as an example (just because you provided, not criticizing you) he uses the word "miriad". We all know what he meant - but the correct spelling is "myriad". Now, i is very possible that he has heard the word "myriad" used on multiple occasion, and he clearly knows the intent of the word. But it is also very possible that he has never SEEN the word before - so how is he supposed to spell it correctly? Now somebody might say "use the spell check." But does he even have a browser hat supports the spell checking? If he does, he may be typing while looking at the keyboard and not staring at the screen. Holding that against him? Seems quite trivial when his point was clearly made.
As I keep reiterating - posts can be acceptable if they clearly add to the conversation or discussion. I am completely for comments that are made to crack a joke that may or may not clearly represent the situation at hand. But when a post is made to correct spelling or grammar AND it holds no purpose whatsoever (e.g. doesn't help to clarify the original intent) then it is out of line and takes away from the purpose of discussion.
We can continue to make logical arguments against the "spelling and grammar" police, but if they do it to be jerks they will never change and we only empower them more. There are two simple methods to handle them: ignore them, or ban them.