Semaphore and Morse users can follow proper English too, but I wouldn't want to carry on conversations that way. 
That aside, when SEE users follow "proper English", they are using the English language. When ASL users follow "proper ASL", they are using American Sign Language. English and ASL are two separate languages, just like English and French are two separate languages.
English has various modes of transmission--spoken, printed, brailed, tapped out in Morse, signaled in semaphore, or signed manually in SEE. They are all representations of one language--English. They are NOT individual languages.
