By law, they ask you to sign to leave a society/organization when, by the same law, they do not ask you to sign to join it.
Imagine being in court with a representative of the JWs.
The judge asks you: "Are you a JW?".
You could respond: "Let them prove it!".
Question: How would they prove it? With a card filled out by them with a baptism date written by them, when you might have still been a minor without your signature (which as a minor had no legal value) and without your parents' signatures? Decades might have passed: where and how would they retrieve the evidence?
So any society/organization could claim with a card written by them that I am one of their members: would that be sufficient for a judge? I doubt it!
But a DA letter becomes the signed proof that you were a member, instead!
In the end, though, everyone is free to do as they see fit!
If I am wrong, correct me, please.