Anecdotally: a couple of years after 9/11, when I was a student, some of my friends who travelled to the US mysteriously disappeared for a few days. Turns out someone in the friend group had done some hackery type things (I won't go into more details to preserve their anonymity), and they were basically "detained" and interrogated for several days without being able to notify anyone, including any potential lawyers (not that students have lawyers on speed dial, but whatever). The culprit wasn't even among the people who were detained. No arrests were ever made. Just some good old scare tactics against teenagers.
Basically: behaviour at US borders has been iffy for a lot longer than some folks might think.
Yeah, recent news are essentially raising this from "crossing US border is dangerous, prepare yourself" to "US border guards got a quota of terror to inflict, do you really want to gamble?"
I'd like to provide a counterargument that it's not "iffy" to detain and question people who you're not sure sure should be allowed into the country at a point of entry, and that basically every country on the planet does it.
Non-citizens at US points of entry have very limited constitutional protection. SCOTUS has consistently held that the federal government has broad authority over immigration and border control. Basically nobody has a 1st or 2nd amendment protection at a border crossing, and non-citizens have further-restricted 4th and 5th amendment protections among others.
Border agents do not need any level of suspicion or probably cause to search your person or your effects. Failing to answer questions can result in entry being denied. US v. Ramsey held that everyone, citizen or not, has no inherent right to enter the US at a particular point of entry on a particular date and time and that basically any search is "reasonable" due to national security and law enforcement needs. That ruling was half a century ago.
Shaughnessy v. US ex rel. Mezei (1953) held that even a lawful resident who is re-entering the country after an absence can be denied re-entry without a hearing as long as that denial is lawful. Mezei lived in the US as a lawful immigrant from 1923 to 1948 then went back to Hungary for just over a year and a half. A 1924 law classified him as an "excludable alien" when he returned in 1950 he was permanently barred from re-entry. This was before LPR status was codified so I imagine there is more relevant case law to that classification specifically.
SCOTUS has consistently held multiple distinctions between citizens and non-citizens at the border: Citizens have an absolute right to enter the country, non-citizens (including LPRs) do not. Everyone loses most 4A protections at points of entry, but citizens have a reasonableness bar that non-citizens do not (US v. Montoya de Hernandez 1985, US v. Flores-Montano 2004). Citizens still enjoy due process while non-citizens do not (Shaughnessy again, Kwong Hai Chew v. Colding 1953). Citizenship ensures someone is not in a legal limbo status of being detained unreasonably or indefinitely (Boumediene v. Bush 2008), a non-citizen denied entry without the means to leave is basically stuck there. Citizens are presumed able to enter the country and have faster processing, all non-citizens including LPRs must prove admissibility every time.
So there's a century (or more) of case law supporting what some might call extreme power on the part of the federal government to deny non-citizens entry at any port of entry, for any or no reason. But what it boils down to is whether there are any countries in the world that don't have this policy? There is no country in the world where as a non-citizen I enjoy the same rights and legal recourse as a citizen if I am denied entry, and no country where it is not on me to affirmatively prove to the border agent(s) that I am legally permitted entry. It is always a privilege to enter a country other than your own.
Edit: At the risk of breaking guidelines and making for boring reading, I have to question the odds of someone being able to read this comment in ~30 seconds, process the argument, and decide its worth a downvote vs. "oh I don't like this first sentence."
It's probably more like "your first sentence doesn't address the GP's situation at all, so the rest of the post is just gonna be grandstanding without foundation". Which happens to be a correct assessment of your post.
Specifically, "disappeared for a few days" is not at all what basically every country on the planet does.
I'm very clearly responding to the oversimplifying final sentence and I cited several instances where non-citizens can indeed be held for days without violating US laws.
The GP frames this as the US doing something nobody else does, which is objectively false, and even if his specific example is an egregious violation of someone's rights I'm sure if we looked through the last 25 years of immigration detentions for other countries we could pick out something equally upsetting from each one.
Most of your comment is spent arguing about "can question and deny entry", which is missing the point by a mile.
Also, things can be legal and iffy at the same time (indeed, such wide-ranging powers basically invite that, since they give wide latitude to go overboard in cases that do not deserve it).
That is very much not what I said. Additionally: some of recent people who've been detained are "criminals" as well, a missing document, a tattoo-gun there on a tourist visum, etc. Doesn't make it right to treat them like utter garbage or to make their rights disappear.
Basically: behaviour at US borders has been iffy for a lot longer than some folks might think.