So they even need a warrant? I was under the impression that no US constitutional protections apply to foreigners, and when entering the country you need a visa or equivalent preauthorisation, and there you certainly agree they can do whatever they want with you.

That sounds mostly right to me.

I think many constitutional protections do apply in case the USA is prosecuting an individual, eg even as a non American you could take the 5th if an American court was trying to convict you.

However, when you're asking to enter the country as a non US citizen your options are essentially to do whatever the border services agents ask you to do or turn around and go home.

...or even just spent the time to do it. But, not too surprising that they don't want to divulge everything that led them to him.

According to another article they arrested / detained several other people during this bust. I am guessing an inside agent got them to meet up. Only Coelho was stupid enough to have his devices unlocked / easily scoured. Using his admin email didn't help. Who even does that? Even my 75 year old mom knew to use her trash email for signing up for crap.