I think the only way to interpret this that's even remotely practical to implement is that while they have the right to ask anything of anyone, in practice, it

I think the only way to interpret this that's even remotely practical to implement is that while they have the right to ask anything of anyone, in practice, it will likely be targeted. If they're asking, they probably already know quite a bit about you, including whether they think you match some social media profile they've red-flagged. I'm certainly not suggesting that Trump isn't stupid enough to think having CBP attempting to guess whether 800,000 grandmothers every hour are lying about not knowing what Facebook is would be a plan he could seamlessly roll out, but I think any such plan would crater pretty spectacularly.