Sort of gonzo thinking, the Army leadership spent a lot of money training, equipping their Soldiers, and would spend a lot of money deploying them across the seas. And once they got their troops abroad, they were more concerned that their Armies not shoot too many bullets at the Enemy, because of the costs of supply!
Not entirely gonzo, if you understand the rationale of the day. And the cost of supply was more a peacetime issue, when combat looms, its the capacity of supply, not the cost, that is paramount.
Outside of combat, things change slowly, both equipment, and attitudes. Technology makes advances, but tactics (and training) to take full advantage of technological advances often moves with glacial slowness, until the shooting actually starts.
The magazine cutoff is a classic example, something that seems reasonable, even prudent, until real world experience proved it to be an unneeded feature.
Remember that it came about during the days well before machineguns, beaten zones, and full auto fire support. Soldiers were trained to fire individual aimed shots, and area suppressive fire was done by volley fire of units of riflemen. (and artillery).
The idea of keeping the full magazine "in reserve" to repel an attack does make sense, if you consider the mindset of the time, which "grew up" with single shot breechloaders being the pinnacle of rifle firepower. With that established, now along comes rifles that hold 5 shots!!!! awesome firepower!!
Not to us, today, but back then, quite a bit. Keeping that firepower in reserve until actually needed did make a bit of sense. Until actual combat (and further tech advances, like automatic weapons) showed that it didn't make sense any more.
Cost matters a lot in peacetime, probably more than any other single factory, especially when money is tight. We got the M1 Garand in .30-06, instead of the .276 Pedersen, simply because the cost of changing to a new round was deemed excessive.
When you get into combat, especially overseas, supply capacity becomes more important than the cost of the items being supplied. We always find a way to pay for them during war, what's more important is that we deliver enough of what's needed to the guys who need it, WHERE they need it, and when they need it.
And, just what is the stuff they need?? History abounds with both right, and (in hindsight) wrong decisions about that, in ALL fields.