Why do people go on about spelling?

Why do people always go on about spelling. I cant spell but its not my fault that i make mistakes i just cant spell the words cos i am dyslexic (sp) I cant spell that one ether. I not been on here for long in already 2 people have made me feel realy bad cos I cant spell. why do people do that they go on bout spelling when they dont know why the persons spelled the words wrong. In i use slang loads to try hide that i cant spell in people go on bout that to not every one can spell good or use gramer right so people whould just shut up about it