"Some airlines, like Continental and Frontier, will require a US passport as they don't want to mess with the background checks required for non citizens, including Green Card holders. It's expensive and time consuming, needed for all future training events too."
It's also illegal but most of all totally incorrect...
I know 2 pilot at Frontier and 5 at CAL who are yet to obtain a US passport... Know several more that got a US passport many years after being employed there...
I've worked with recruitment issues at several smaller airlines but the process is the same at all airlines. Your statement is simply incorrect. Not sure why you're spreading this false information?
Some airlines might have specific requirements if they have lots of DOT or postal contract flying but even then a US passport is not required. FDX for example wants you to have physically lived in the US 5 years prior to being employed there due to their huge postal contract...