Do you think it’s unfair for a company to deny a retired Army E-5 from working for their company because he has bad credit?

I don't know what being retired Army has to do with it. If the company policy is no bad credit, that's what it is. If I was running a financial institution or a cash heavy business, I wouldn't want someone with bad credit working there either.
Army has the same rules for security clearances. Would you ask, "Do you think it's fair for an Army E5 to be denied a clearance because of bad credit"? When you say it like that it sounds ridiculous. Being in the Army doesn't make anyone automatically trustworthy or employable.

No, it is not unfair. All companies have hiring rules that have to be followed.

Nope, not at all unfair!!! Quite the opposite actually. Smart!!!!!