Earlier quoted context omitted.
The idea is that they cannot in fact make you turn travel mode off, because travel mode doesn't turn off. It's a time lock. The point is that while locked, Facebook and Google Mail are still usable; they just don't have your whole history available on them. If we stipulate that your second paragraph is true, then travel mode is in fact a complete countermeasure to that behavior. (I don't think it's true).
Why wouldn't they just detain you until the time runs out?
In contrast, suspending 4th amendment at border has been made lawful by the courts.