Why not? It's not like they said "ten hut" anyways. They're in the battlefield, not the drill field. Only reason to say to not include the Marines is because they were put to action against the Spring Offensive in 1918. However, the U.S. Army had the 1st Division on the front line in October 1917 near Nancy.
Doesn't mean they should get 'two' slots though. For 'Other Allied' forces, there should be about 2 slots for Canada like the Royal Newfoundland Regiment and the The Royal Canadian Regiment. Australia should get 2 for example the 1st Battalion and the New South Wales or even add a New Zealand detachment like the Auckland Battalion.
Then seeing as there are 5 troop slots on the page, you can have one for Americans most famous troops in WWI, such as the US MARINES OORAH and all that stuff. But I know that being an American, you want to see lots of American units but to be fair, seeing as this mod is supposed to be set in a 1916 to early 1917 time frame as said on the first page, the vast majority of units would be non-American. But as you said, the 1st Division was on the frontline October 1917, meaning there shouldn't really be any Americans in the game in the firstplace as its set 'early 1917' so having at least one unit in there should be considered lucky, let alone 2 or 3! xD