By analyzing the data, you could theoretically figure out patrol and supply convoy routes, and make educated guesses as to where on these bases soldiers eat, sleep, etc. That's a lot of useful information for someone planning an attack. You could also track individuals, potentially important ones. One researcher claimed they tracked a French soldier's entire overseas deployment and subsequent return home.
This wasn't an evil ploy by a terrorist cell in league with Big Fitness; you can turn that data tracking off. It's just that no one even thought about it until someone finally pointed out that it was a huge security issue. American rules for fitness trackers in the military are now being "refined," which we assume is PR speak for "Goddammit, turn that s**t off." But it's only a matter of time until another seemingly innocuous technology accidentally gives away state secrets.