If you're a nonresident, the CBP officer has full discretion whether to let you in or not, and you have no right of appeal. So when the officer asks you a question, answering it is not really optional.
True. But it will be LISTED as optional on the form.
If you get an interviewed by the thugs at the border it will definitely be non-optional for all intents and purposes.
The whole thing is messed up either way. They'll no doubt just start asking you to log into your social media accounts so they can go through your personal shit, I've had US border agents go through my phone and ask me idiotic questions about the photos on it (all family vacation photos).
Entering the US or UK is a pretty terrible experience all around. They're rude for no reason (and it has nothing to do with their job), overly invasion, and their entire justification is based on their own gut rather than fact (inc. ten year bans).
PS - I have never been rejected at any border. Just been hassled a few times.
