As far as I can tell, the rules regarding US passport issuance allow legal guardians to act in place of parents where necessary. This makes sense, as legal guardians, with the proper paperwork, are generally entitled to act as parents, and a regulation that prohibited guardians from doing so for passports would be impractical.
22 CFR 51.28 gives the actual legal rules for passports for minors. The phrase "or legal guardian" is used throughout. (b)(2), which talks about passports for 16 and 17 year olds, says "The passport authorizing officer may at any time require a minor 16 years of age and above to submit the notarized consent of a parent, a legal guardian, or a person in loco parentis to the issuance of the passport." It seems that the consent of a legal guardian can substitute for that of a parent in this case.