Wouldn't you just put that in your privacy policy so when some bug makes it in (perhaps in upstream code or the OS itself) that causes location tracking to continue happening when the app is closed, you're not open to liability?
It is difficult to discern "paranoid" from "evil" when reading legal documents. Unfortunately being paranoid makes them look evil. Maybe they are, maybe they aren't, but there is more than one way of looking at this.
It seems difficult for something like that to accidentally happen, seeing as background location tracking requires explicit permission requested through APIs provided by the OS.
I wonder if the fear is that you might end up having less than ethical (rogue?) employees that might think they're helping the company by using some form of workaround to track users. I'm not sure if that's overly paranoid or just the right amount of paranoia, but I can imagine the fear.
It is difficult to discern "paranoid" from "evil" when reading legal documents. Unfortunately being paranoid makes them look evil. Maybe they are, maybe they aren't, but there is more than one way of looking at this.