Do you honestly not understand why people nag you about your bad grammar, or are you just ranting?
It's extremely simple. Using bad grammar makes you appear stupid. There. It's as simple as that. You may be the smartest kid in school, but if you continually make yourself look illiterate, then that will become your reputation. Remember, when you're online, people judge you by the way you express yourself in writing because that's all they have to go by.
What's more, the bad habits you pick up now will only be more deeply ingrained with time. One day you'll be in the "real" world having to earn a living. If you think it's not important, just wait until you have to write a cover letter or a report to the boss or an interoffice memo. If you look stupid, your stock will plummet and it won't be easy to recover. Think nobody has ever lost a chance at a job, or a promotion, because they made themselves appear to be of low-to-average I.Q., or too lazy to be professional? Think again.
Even in your question, you didn't use any punctuation at all until the ending period, and it was sorely needed. You didn't capitalize anything that should have been capitalized, not even the first word. For somebody who's so concerned about people ignoring your bad grammar, you sure do go around pushing their buttons.
Proud and unregenerate member of the Grammar Police.
By the way, calling somebody a "Nazi" is indescribably offensive, and if that's a sample of the way you express yourself, then I'm not surprised you have such a hard time getting along with people in this regard.