Dude you're being a bit shortsighted with your own argument.

Sure not all of that military space budget is applicable to helping towards a mars mission, but that doesn't mean a fair amount of it isn't.

Using your taco truck example, its like they are buying a crate motor for their food truck. The taco guy(NASA) doesn't have to go design a motor because they are allowed to buy it from a car company(the military) that's job includes designing and building engines, among other things.

Sure the car company spends lots of money building cars and other things besides engine design, but that doesn't mean that by allowing the taco truck to ignore engines and focus on what they are good at: delicious tacos(science experiments, space probes, etc) that the car company doesn't help the taco truck stay in business.

If the taco guy stopped selling tacos, studied to become an engineer, designed a motor, paid for the machinery to make the motor, built the motor, fixed the inevitable flaws in the end product, tested it, etc, do you think he would still be calling himself a Taco Truck worker, or that his homemade engine would be as reliable as a mass produced one?

Conversely, does the car company care about how to make a delicious taco? As a business it isn't their goal, but lets say the engineering department loves taco Tuesdays. Unfortunately Pedro's taco truck breaks down! They decide to help out Pedro by getting him a replacement motor at cost, helping him install it after hours, then Pedro gives him his taco recipes in return. You won't see it on their balance sheet but you sure as fuck better bet they are making those tacos for dinner, and Pedro is still in business.

If you think NASA and military research don't go together like flies on shit you don't know what NASA does. They work hand in hand with the military in aerospace design and testing, and the military plays a large part in space design and testing. Its a mutually beneficial relationship. Look at projects like the x-37b, previously a NASA project, moved over to the air force, and now its broken several records for unmanned, re-useable spacecraft. I highly doubt the Air Force hasn't shared a single bit of information learned from that project back with NASA.