Explanation: When America entered WW1, we brought a very large number of pump-action shotguns to trench warfare, many of which were capable of slam-firing at a time when most long guns needed a few seconds of movement to fire another shot. Shotguns, furthermore, are often loaded with, well, shot, which has a bit of a spread and can take... nasty chunks out of people at close-range. While it's not so great at a distance, it's fantastic for, say... narrow muddy trenches filled with enemy targets.
Imperial Germany did not like this, and formally registered a complaint that the use of shotguns was inhumane (in the war that introduced chlorine and mustard gas attacks). "lol" said everyone, "lmao"
Interestingly enough, Germany was actually known for its very high quality hunting shotguns, and actually did use shotguns in WW1 before the USA came in... for soldiers hunting food, and eliminating enemy messenger pigeons. Traditional German hunting shotguns tend to be either single-shot - and you don't want to be caught reloading with a bunch of angry Brits/Frenchmen with bayonets a handful of meters away - or multi-barrel, making them a bit heavy and unwieldy for the highly-mobile combat of trench raids.