They just use those spellings to sound hip or cool or funny or to exaggerate. And sadly, many native English speakers have poor grammar. So some of the grammar mistakes are because they don't know any better, but some are also just for fun.
For instance, I'll use the word "ain't" sometimes to exaggerate what I'm saying, to draw attention to my speech. Of course, it's bad grammar, but my friends know that I know the difference and I'm doing it on purpose.