United Airlines defended a gate agent’s decision Sunday to bar two teenage girls from a flight for wearing leggings, saying that the young passengers were traveling on a company travel pass and were in violation of the airline’s dress code.

To be clear, United says, if you’re paying for your flight, “your leggings are welcome.”

The incident reported Sunday by a traveler at Denver International Airport, set off scores of angry and mocking tweets. But it also ignited fury among airline travel pass users, who say the girls taking advantage of the employee benefit should have known better.

Late Sunday, United provided the following explanation:

“Let us take a moment to explain today’s news:

“We care about the way we present ourselves to you, our customers, as we believe that is part of the experience on board our flights. One of the benefits of working for an airline is that our employees are able to travel the world. Even better, they can extend this privilege to a select number of what we call ‘pass riders.’ These are relatives or friends who also receive the benefit of free or heavily discounted air travel – on our airline as well as on airlines around the world where we have mutual agreements in place for employees and pass riders.

“When taking advantage of this benefit, all employees and pass riders are considered representatives of United. And like most companies, we have a dress code that we ask employees and pass riders to follow. The passengers this morning were United pass riders and not in compliance with our dress code for company benefit travel. We regularly remind our employees that when they place a family member or friend on a flight for free as a standby passenger, they need to follow our dress code.

“To our regular customers, your leggings are welcome.”

Early Sunday, however, when fellow passenger Shannon Watts tweeted the incident, it was not known that the two girls were on a company pass. Besides the two teenagers, Watts said she saw a younger girl, around 10 years old, who was also asked to put on a dress over her leggings to board the flight from Denver to Minneapolis.

United’s response then, via Twitter, was that under its passenger contract, it has the “right to refuse passengers who are not properly clothed.” And added, “This is left to the discretion of the agents.”

The Twitterverse went nuts over what is considered “properly clothed.”

But forget about the passenger agreement, United said later. This was about the airline’s internal policy, which requires employees, and anyone using their free travel pass, to abide by a dress code. That dress code says spandex pants are “unacceptable,” as are mini skirts and shorts that are shorter than 3 inches above the knee.

Lots of companies set dress codes. But some critics said even if United’s policy only applies to non-revenue travelers, how do fellow passengers know that? Others say in an era where leggings are made for casual wear – and even acceptable as part of a work outfit when well-accessorized – the policy is outdated and too strict. Others said the policy is sexist because it singles out attire worn primarily by women.

United isn’t the only airline that has a dress code for relatives and friends of employees traveling on the company dime. Policies from several major U.S. carriers listed on the website FlyZed, which keeps track of airline employee policies, show that airlines generally require travel pass users from wearing clothing with holes or anything that has offensive or derogatory words or graphics. They ask them to be “well-groomed, neat and clean.”

American Airlines’s policy allows shorts in the economy cabin, but prohibits anything revealing “extreme mini-skirts, halter and bra-tops, sheer and see through clothing.”

Southwest says it has a “relaxed and casual” dress code, but don’t bring “low-cut, skimpy, revealing clothing, short shorts or gym shorts.” There is no mention of leggings.

JetBlue prohibits leggings but says jogging suits are acceptable.