How easy is it for an airline to make its loyal customers happy? Just ask United Airlines. The members of its Mileage Plus program have been happy campers for 10 days, spending their frequent-flier miles like there is no tomorrow.

Many of those travelers waited for that opportunity for months, if not years. They had joined Mileage Plus and mounted sizable accounts with the assumption that they would be able to redeem their miles for flights on the Star Alliance, the global network of 24 carriers of which United is a founding member. But often in the past few years, when they tried to book “awards” on Singapore Airlines, Germany’s Lufthansa or Asiana of South Korea, among others, finding availability was a Herculean task. Many travelers were angry…