Again we are talking 'deterrent'...and what 'pictures' any sticker on a car 'paints in the tailgater's mind' _that might cause some refrain out of concern that the person in the car is no pushover thus biting more than the tailgater can chew.
If no 'doubt' is introduced in a punk ass moron tailgating you that he might be in a big mess of trouble...such as possibly pissing off a Marine/vet who would welcome the chance of burying him alive...then the average car on the road will be seen as 'White bread' by the tailgaters.
Could a USMC decal invite trouble instead of sending an avoidance message? Sure...all is possible...
But usually a Marine behind the wheel of a car is not perceived as a slouch...thus easy to !@@## with.
I don't know. But all my Marine friends who have USMC decals report no problems for the most part, except for the occasional punk who will 'buzz' by but has no balls to really take that car on.