ASL is not a written or spoken language, it's a visual language. Of course it has a difference sentence structure than English. It's not English, and shouldn't be compared to English. ASL is recognized as a foreign language in many colleges and high schools now. One myth about ASL is it's just signs in English word order.
English is a second language to most Deaf people. Online, they communicate mostly through video blogs, since they can see each other.
Are all written and spoken languages the same? No. Every country has it's own sign language just the same as the spoken language. How could there possibly be a universal sign language? (Is there a universal spoken language?) It's about as ridiculous as expecting a sign language interepreter to read Braille! (which happens quite often!)
The Deaf write in ASL GLOSS, not English. They should be able to learn first in their first language, ASL before being forced to learn English, which is their second language. Those who are functionally illiterate have parents who did not instill education in them as a very young child when language learning was very important.
One other note: It's much easier for a hearing person to learn ASL than it is for a deaf person to learn to speak!