This whole "well, they need some space around the border to operate" seems to be a red herring. No one's objecting CBP from setting up checkpoint buildings at the US border. They've been doing that since 1789. The extension of this space to 100miles is a travesty. Make it 0.5Mi so CBP can't waltz around random bus stations, and no one will complain.
[1] https://www.wweek.com/news/courts/2019/01/28/border-patrol-c...
It's also reasonable for CBP agents to be able to enforce border-related laws anywhere near the border, subject to the usual law enforcement requirements of reasonable suspicion of a crime based on objective evidence for brief detention, and probable cause for any kind of search.
What I find unreasonable is any kind of checkpoint where a substantial number of people passing the point are unlikely to have crossed a border. For example, there's a permanent checkpoint on I10 about 15 miles west of Las Cruces, NM. It's over 30 miles from the border with Mexico in a straight line, and much farther by road. Most people passing this point are traveling between two locations in the US. Everyone is stopped and questioned, and while the questioning is usually not very invasive, it's done with zero individualized suspicion.
I’m not saying that’s a good place for a border check station. I’ve always thought it was silly, but it’s been there for a while.
In this particular case, the 100 mile debate is irrelevant. The argument here is not that CBP doesn't have the right to do a warrantless search at the border, it's that that right does not extend to searching the contents of a mobile device. Border-crossings are an "exception" to the fourth amendment, and that exception is a "narrow" exception that only applies to what is needed in order to balance the governments interests (here, securing our border against illegal immigration and importing goods not permitted) against the rights of the individual. The search is scoped, and should not exceed that scope; the argument is that searching the contents of electronic devices exceed that, since CBP has no business there.
[1]: https://en.wikipedia.org/wiki/United_States_Border_Patrol_in...