I knew at some level that American Sign Language is not the same thing as English. But I hadn't realized what that means for those who are deaf before language is acquired:
Capital D "Deaf Culture" is markedly closed to the non-deaf, but even more surprising is closed and in many cases actively hostile to anyone, hearing or deaf, who promotes communication in any way but ASL. There is nothing wrong with using ASL as a language, except for the fact that there is no written form. And Deaf people live and work and sign contracts in a world with written language and that written language is not ASL. Being fluent in ASL doesn't give one a command of English. Anyone using ASL to communicate must be bilingual to operate outside Deaf Culture.
Prelingually deafened children raised using ASL or another of the signed English systems (which keep trying to force ASL to be more like English) have roughly a 10% success rate at reading English (or any other traditionally spoken language) on grade level above the 4th grade. Reading the writing of the average Deaf adult is like reading an English paper written by a foreign student, as they are both writing in a foreign language. Imagine being raised speaking English and only ever learning to read in Spanish. Some do remarkably well, but the odds are stacked against them. It's extremely hard for them to succeed in standard high school and college courses when they are not fluent in English.
I've always wondered why the deaf were so poorly integrated into the standard workplace; it seems as if they should have much wider opportunities than, say, the blind, or people with mental disabilities. But of course in English, at least, reading is tightly connected to speaking; you don't learn to read a phonic language well unless you can also speak it.