I don't see how Americans could fight for their country in WWI, because their country was not reachable directly with war that time. (Not counting possible Mexican intervention)
Even in WW2, USA wasn't reachable, when it gained arbitrtary position to Europe as GB was in WWI. And even British decision to participate in WWI was very unstable in beginning... How you can say "They were fighting for their own country" after this?
And, I didn't say "homeland", I said country in which volunteers (or their parents) was born.

More precisely, I meant volunteers, including all Americans in service of european air forces, years before USA officially entered WWI

Don't wanna to debate this topic anymore, because it needs deepest researches with sources, hardly understandable now, because of difference in people's psychology between 1900's, 1950's and current time.