They can Crossfit, they just can't OPSEC.

In a memo posted by Deputy Defense Secretary Patrick Shanahan on Monday, the Pentagon revealed the new rules, which will prevent members of the US military on active duty from using fitness trackers, any applications in mobile devices which use GPS, as well as any "other devices and apps that pinpoint and track the location of individuals." The Pentagon says that the information stored by GPS-based services can be uploaded to servers which then may be shared with third-parties -- and therein lies the risk.
https://www.zdnet.com/article/pentag...ices-with-gps/