I think they have the right to "ask" just about anything they want. They are federal law enforcement agents, they can ask questions unrelated to your luggage. If they ask for a passenger's gmail account, and the passenger gives it to them, and they have a look at the emails, I don't THINK that's a constitutional violation. As a general rule, police officers can "ask" you do/give them anything and if you consent, there's no constitutional violation.

If someone has contrary legal authority saying this is a constitutional violation, I'd be interested in reading it.

Unfortunately, many people don't understand the nuances between when an officer is ordering them to do something and when that same guy (with a badge, gun, and uniform) who is detaining them is merely "requesting" their voluntary consent for something. Compounding the problem is that cops are deliberately vague about this. A CBP officer won't take out your phone and say "I'd like you to voluntarily enter your password, but of course you have the absolute right to refuse." Instead, he'll just say "Could you enter the password?" which a layperson won't know he can refuse. Then the officer will report that the passenger voluntarily entered it of his own freewill.