Here is the rules for minors traveling alone. Seems pretty clear to me. The parents did not follow the rules that are posted under unaccompanied minors at USAirways.com.
"Children traveling without a parent or legal guardian are considered unaccompanied minors.
Children under 5 may not travel without parent or legal guardian.
Children 5-14 may travel unaccompanied on non-stop flights for an additional $50 fee each way. US Airways assistance is required.
Children 15-17 may travel unaccompanied on non-stop and connecting flights without US Airways assistance.
Children 15-17 who require US Airways assistance may travel unaccompanied on non-stop flights only. There is an additional $50 fee each way. Uniformed employees provide assistance for children traveling alone from the point of departure to their final destination. Personnel are not qualified to provide certain types of medical assistance, and are not permitted to administer or dispense medication. Please take these points into consideration if the child requires specialized care.
If an unaccompanied minor is traveling on a flight operated by one of our Star Alliance or codeshare partners, you must contact the carrier directly for their policies on unaccompanied minors.
When a young adult traveling alone checks in on the day of departure, a notation will be added in our computer system which will alert other US Airways personnel of the young adult's presence on the flight. This notation will aid US Airways personnel in providing assistance to the young adult in the event of an irregularity."
Does this mean that US Airways does not escort UM on connecting flights at all? If so, I guess that's just another casaulty of the cutbacks at "full service" LCC.
By the way, the US portion of the trip, SAN-PHX was non-stop, so I don't know why you're blaming the parents. They wanted someone to meet the kids in PHX and tell them their new gate information. US Airways failed.
They didn't get lost, though, so the article was too harsh in saying that. They were delayed overnight, not lost; big, big difference.