If your contract doesn't say, "You can't put sawdust in the brownies," there will be a contractor who says, "What? You didn't say I couldn't."
You might laugh. You might suppose there's some common understanding of quality that everyone can deal with. Especially when it comes to brownies. To some degree, I'd agree: it's a defensive attempt to avoid problems encountered in the past, and maybe an overreaction.
The thing is, though, with weapons systems, the equivalent of putting sawdust in brownies happens all the time. The contractor doesn't live in Alaska, or on a carrier deck, where the thing will be deployed, and if the contract doesn't specifically mention cold or fungus or salt corrosion, it'll get overlooked. Worse, the contractor really has no experience in a combat zone, and has no idea what sort of abuse their equipment will undergo. The contract has to say.
The defensiveness comes partly from the fact that these systems get a single iteration. There's some back and forth during testing and verification, and there are always contracts for upgrades and improvements. But by and large, you really need things to work right the first time. Hence the heavy reliance on contracts and standards.
They're like an API telling the civilian contractor what's required in a battlefield environment. He really doesn't know things that soldiers consider obvious.