Personally, I think the entire mindset is wrong, most people speak the language that they're most comfortable with, which it seems like they were doing. Now, if they were talking at you, and snickering and such, then sure it's impolite. But that should still be their right to speak the language that they are most comfortable speaking.
The fact that it is the United States Army is one of the main reasons why speaking other languages should be okay, the United States is built upon diversity (or so they say) so different languages should be embraced, not looked down upon.
Just because someone now lives in the states, they shouldn't have to compromise their heritage. It's wrong of Americans to expect them to do so.