In my opinion, this is a myth made up by dealers a decade ago to justify the fakes. Think about the global politics, German officers weren't going to buy French helmets. And where would they buy them? Germany was exporting arms and equipment, so I can't imagine it would have been acceptable. I imagine Joe and Gus would have more insight than I do on this matter. The British hat makers did sell helmets in their colonies, and I'm sure the same was true with the French, but were German officers traveling through those colonies?
As for the American helmets, I seriously doubt it. Again, where would the helmets have been purchased? The U.S. wasn't exactly exporting these things around the world. The only countries that I have confirmed used American-made helmets were Mexico, strange as it seems, Canada (in likely minimal numbers), and Denmark! Yes, Denmark... it still had the Virgin Islands until 1917, and my research found that the Danes stationed there acquired a small handful of American helmets. I bet the number is fewer than a couple dozen. Clive Law and I spoke about the Canadians using American helmets. I initially doubted they were real, thinking it was a case of helmets with badges added. However, he suggested that Canada was facing a need and may have acquired a small batch. As for Mexico, I don't know when/how they obtained the helmets, but it was sometime in the late 19th century. Mexico bought surplus arms from the U.S., so maybe it was part of a larger deal. I've seen the photos of Mexicans wearing the helmets.
Here is the other part of it: all of these French/American helmets appear to be in excellent shape. You're telling me that an officer bought a helmet and then didn't need it?
Again, Joe, Gus, and others might have better numbers, but we need to understand that the German colonial forces were relatively small. Did they really need to purchase these foreign helmets, which amazingly survived?