I think literacy in the US is at an all time low. It's pretty amazing to see some of the stuff written even by recent college graduates.

I suspect that a lot of the people writing the text are simply unaware of the errors they are making. Certainly some of them are just oversights, but in a lot of cases, I think the writers have no idea that what they're writing is incorrect.

And as far as the surfers go, I think a lot of people never even notice; they either don't read the text, are too busy jerking off to pay attention, or just don't care. But, like you, I'm one of those who does find the grammatical and syntactic errors to be a major detraction to a site, even if it's otherwise well done.