Why do so many people have a difficult time using basic English grammar on this site?

Firstly, I'd question how you determine native vs. non-native origin. Secondly, I'd suggest that many "native" English writers treat this forum as they do email. That is, they drop any concern for grammatical correctness. Correct usage is not intuitive for many folks, and it's easy to let it go as a priority when one quickly crafts questions and answers. I, myself, tend to be somewhat of a grammar nazi. Yet, I do not find myself terribly concerned about this style of communication you note. I think those who decry such usage are not willing to accept the idea that both language and modes of communication necessarily evolve.

It's like learning another language, I find. When you speak your mother tongue, you know all the slang, and can adequetly put your point across in a quicker fashion. But when you are speaking a language that is not your first, people tend to be more proper, because that is the first way we are taught to speak. In proper, complete thoughts, proper puntuation etc.

i think the reason no one can spell on this site is because the average age must be about 10 years old

We don't have a problem with gremmer. Your just way two picky. Grammer don't affect anything it makes writin way two hard anyway.

Because such things aren't important in American schools anymore. They'd rather teach things like "My two moms" or teach that Islam is a religion of peace no matter how muc Islamist violence goes on in the world around us. The American education system has gone from teaching the three r's to being surrogate mother.

Let's use an analogy. Have you ever played Pictionary? The disparity in talent when playing that game is shocking. Suppose you have to draw a house. One person might produce a Monet version of a house in 30 seconds, while another person might be lucky to draw three lines. The person who drew the Monet might have atrocious grammar skills, while the stick-figure person might be a best-selling novelist. I think there is some natural ability involved with writing, but I also think that with practice people can improve their writing skills.

I really do not know how you figure out native English speakers from non-native speakers, but if it is as you say, the most logical explanation I can think of is that native speakers first learn from hearing and then learn how to write it. The non-native speakers (most of them) take courses that teaches the language by reading and writing it. That way non-native speakers get more acquainted with the correct form of writing of the language than native speakers. I also think this must be common for any language!!

i do not no y so mani people have trouble with grammar, it does not bode good for us :P

After you've spoken English all your life, you become more comfortable with it an also more lazy. Non-native English speakers make more of an effort when they start learning this strange, different language because it's so easy to be misunderstood. I am experiencing this too because I'm learning Vietnamese. It is a difficult language and I've noticed the difference between the way I speak it and the way the native Vietnamese do. They are indeed a bit lazier in the way they pronounce words as well as writing them, (as the marks over letters can become ambiguous,) and I make a more concerted effort to be correct so they will understand me. This just has to do with comfort and laziness. Although it's not that they do it out of lacking consideration, it's merely something that grows over time.