It's based on the interpretation of "unreasonable" and while the current rules may be far from perfect I don't think interpreting routine border searches as categorically "unreasonable" is better.
Further, in what way is the content of someone’s cellphone relevant at a border specifically? Inspecting luggage for cocaine at an entry point makes sense, but physical borders mean little in the face of the internet. You might as well just erase the phone before crossing the border.