According to this post on Techdirt, it's because of a potential GPS threat. That's right, GPS threat! The new promotion that Coke is running says that all you need to do is pop the top of a winning can of Coke and the can will send a GPS locator signal out to the prize patrol who will bring your prize to you.
To be a little more clear on the winning process, a red button on the can must be pushed in order for a GPS signal to be sent, but the military doesn't seem to think that bringing any sort of GPS receiver onto a secure installation is an ok idea.
I wonder what they do with all the E911 cell phones with GPS capabilities.