By this logic, why keep anyone around after the SF86 hack[0]? Simple answer: Costs. It's a lot more expensive to lay off everyone with a security clearance (especially, if they're regular employees) and make a whole new batch of people go through the security background checks. We're probably talking billions - not to mention the issue with downtime of no one being around to do anything.
> So, why doesn't the military than treat every soldier who has a mobile phone near to him where the above could cause military problems to be a saboteur (perhaps even with the accusation of being a spy of a hostile nation) who should be charged by a military tribunal?
...because they (they being the military, in general) have - relatively - planned for this in places that they physically control (and the threats levels demand it) by essentially making all buildings that need to be secure giant faraday cages[1].
What that doesn't account for is people outside of the buildings[2], which is probably what they're hoping to address with this change. (Too little, too late, I think - as that data's already out there.)
0 - https://en.wikipedia.org/wiki/2015_Office_of_Personnel_Manag...
1 - https://en.wikipedia.org/wiki/Sensitive_compartmented_inform...
2 - https://www.wired.com/story/phone-data-us-soldiers-spies-nuc...