As I said, I think it is tied to the mail, and the background check required to carry it. A US Gov't clearance requires a credit check, as being heavily in debt or spending way beyond your means and NOT being in debt are "risk" factors. Same reason for the long application, and residence/employment history that you have to fill out. If the airline hauls the mail, and you can't get cleared, that can lead to termination.

For the Frac/135 Operators, you are probably going to be issued a company credit card....

If you have concerns, get your annual free report and check it out.

HTH
Spongebob