Why do some people on here try to tell me I "know" what they mean when I obviously don't?

Instead of rephrasing their postings in a more grammatically correct way so that others can understand, they try to tell me that I knew all along what they meant and they even have the cheek to accuse me of being an ass about it. What's all that about? Wouldn't it be simpler to make their postings correct right from the start?