Firstly, because in general terms, if I can understand the meaning then where's the harm?
Secondly, I would lay myself open to ribald, sarcastic comment should my own grammar / spelling be incorrect and trust me, it would happen sooner rather than later.
However, today, I received a written report explaining why one of my students, a young commissioned officer with a post graduate university background, was unable to hand in an assignment on time and asking for an extension.
The explanation was complex but included a claim of illness, specifically "stomick ake".
Luckily, his understanding of fluid dynamics seems to far outweigh his command of language.