The capitalized word is meant to highlight the cultural and social parts of living in a community with other Deaf people. You also would not write American in the lower case.
Being "deaf" is about the handicap, about the loss of hearing and other problems.
Being "Deaf" is about being a member of a community, about the keen visual sense, about communicating in a Signed Language, and so on.
One example: my wife and I realized that we are a lot more able to read the mood of our children by just looking at them than hearing parents. We just "see through" our children.
Even in hearing people, I'm reminded of those who are not just big readers but are used to reading; those who can skim and still obtain 100% of the writer's meaning as opposed to others who aren't so comfortable, fluent, or quick readers — but are not uneducated, just not accustomed to the medium.
Of course the bookworms then have an easier access to education in written form (and that's why I am both a Dipl. Ing. FH in IT and a Bachelor of Law).
Why not capitalize every Noun like in German
Did you see my careful use of the capital letter?
+it’s almost 100 degrees here