Guessing they need ammo which has been cleared for spaceflight, as you want the ammo go up, potentially stay there for extensive amounts of time, come back down, and still be in working order.
And I guess also not go off if it is heated or shaked too much on its way to space!
Since it's stored very close to the astronauts that shouldn't be a problem. Or rather you'd have a different set of problems if your astronaut compartment was hot and shaky enough to let ammunition go off.
Even if it does go off, as long as the rounds are not chambered it's really not that big of a deal.