The 100 mile "reasonable distance" is used to define where vessels and vehicles may be searched for aliens.
But the warrantless search may only be applied to persons seeking admission for whom an officer has suspicion of reasonable cause for denying the person entry.
Of the 80% of people living within that distance (which is an upper bound, btw; the agents in charge are required to set a bound not to exceed that by taking into account such things as "density of population, possible inconvenience to the traveling public.") almost none can be suspected of being under reasonable cause for denial of entry.
So to do the thing you are fearing, 1) the chief patrol agent has to set the distance to encompass an inappropriately large area in violation of this law, 2) an agent has to stop and search cars randomly, and 3) somehow become suspicious that an occupant is seeking entry and ought to be denied entry, and 4) believe that searching that person's device would reveal information demonstrating that the suspicion is correct.
It's not great, but it's not "80% of Americans can have their devices searched without a warrant".