I see what you're saying. I'm inclined to agree with you that there is a certain grammatical hint that points to deaf person but there is other grammatical hint that points to something else such as a foreigner from certain country because of its different grammatical structure (like... Nigerian scammer).
There are actually many hearies who are quite intelligent but illiterate to a certain degree and perhaps write much worse than deaf people. Guess who? ................ Dyslexic people 
so your argument doesn't really make much sense since you and I are deaf so why aren't we writing like them? This is the proper reason - anybody who has bad writing is because they had a bad education early on. Many children who are born deaf tend to have something else such as learning disability, dyslexia, ADD, etc. but unfortunately - it went unnoticed until it was too late. That's why they were mistreated as "deaf and dumb".
