Bad grammar is everywhere on the internet.
There are several main reasons I can think of for this. Firstly, anyone who wants to use the internet can, and they can do it quickly and easily, without any checking of work. I write this writeup, hit the button, and up it pops. There is no editorial review. No proof-reading as I wait for it to print. A better way of wording it might be 'bad grammar is the halitosis of the general population, and the internet reflects that'.
Secondly, time-to-live. If you're writing a letter to your grandmother, you would take the time to spell check it and proof-read it, because you won't be able to post it until Thursday anyway. You have plenty of time to get everything full and correct. If, on the other hand, you were playing CounterStrike online, and your team has just come under fire, you don't stop moving, move your hands off the control keys, and type a full, well thought out message. There isn't time. Everyone knows what I mean if I use U as a pronoun anyway.
Thirdly, population segmentation. The majority of internet users are young males between the ages of 13 and 30. This is not traditionally a group of people who pride themselves on perfection and excellence. Also, many of them have been to school, and I don't know about where you are, but in most schools I know of, it isn't considered 'cool' to be able to differentiate between Who and Whom.
Finally, the difficulty of prevention. It's fairly easy to program a spellchecker. You just compare the word to a stored dictionary file. Since grammar has a lot of rules, some of which make sense and others which don't (and most programmers are ignorant of these rules anyway... I know I am), there is no easy way to incorporate automated grammar checking into a program.
In conclusion, there is bad grammar on the internet, and it probably won't improve any time soon. Suck it up and drive on, marine.
(See also: Reasons that the English Language is Hard to Learn)