Its not the Batmobile having guns. The Batmobile has almost always had some kind of not-really-nonlethal firepower ( I don't distinguish between guns and missiles, its not meaningful ). Its more. . . having the firepower be very overt and exposed. Usually, you don't see the guns or missiles on the Batmobile unless they are actually being used.
Granted, it could be that they are retractable, but they really don't look it. You'd need CGI to retract those guns, and if that were the case, you'd expect they'd use CGI to *insert* the guns. After all, it'd be kind of silly to put a feature on your prop vehicle that you need to digitally delete almost every scene, when you could instead just digitally insert it for the 1-2 scenes its needed.