[fritztastic]

The active threat of someone intentionally driving through a security checkpoint at a military base is an entirely different context. At the point they're crossing the threshold they've already disregarded very obvious markings, signs, obstacles, and undobtedly commands from the sentries on post. There are a number of scenarios for which there are pre planned responses- deviating from those leaves you facing the UCMJ and you're expected to have a very solid justification for using deadly force.

Shooting at a vehicle not actively causing harm due to a perceived threat because it looks like the vehicle of someone you reasonably expect to be a threat is... well, that's a stretch.

Anecdotally I can say from personal experience there is a lot more restraint being used when guarding people/places relating to national security in place where you can reasonably expect to encounter threats and terrorism than the amount of restraint police here use at civilians domestically.