as an immigrant, I have always opted out of the scanner with undocumented effectiveness and undocumented health effects with no consequences other than a pat down.
This link[0] is admittedly an advert for legal services, but there are a couple of interesting snippets in there such as:
> ... if you have been refused entry into the United States at any point in the past, even for an expired passport, the previous denial is reasoning enough for future denial.
IANAL but my understanding is that for non-residents there's no legal obligation to admit you outside of maybe some of the international refugee conventions (which are unlikely to apply). The law is deliberately discretionary. A refusal to admit can be based purely on "suspicion". You can imagine a CBP officer finding you suspicious (rightly or wrongly) because you refuse to hand over your passwords even if they don't have a legal right to force you to hand them over. They can simply offer you a choice of handing them over or going home.
To be clear: I'm not advocating for this state of affairs, but it does appear to be the case.
[0] https://www.visaplace.com/blog-immigration-law/denied-entry-...
specially nice is the quotes: "and something you experience so infrequently, even if you’re a regular flyer—that you don’t have much to worry about."
such a glorious logical falacy. or they dont know what infrequent and/or regular means.
and yet, the author saw it proper to publish twelve quotes from that person with unbased, uncontested positive remarks. great journalism.