This is what United Released about it a few months ago (end of August), some other airlines followed them:


Back-to-Back Ticketing

As a result of an audit of tickets earlier this year, the company detected a sizeable number of transactions that involve violations of its tariff rules.” The prohibited practice is commonly referred to as “back-to-back ticketing.” Flight coupons across tickets are used in an order other than that in which they were issued. Ultimately, it results in a wrongful effort to obtain a discount to which the customer is not entitled.

This resulted in travel agencies receiving formal notification in June of United’s plan to enforce its ticketing rules, including suspending or terminating the agency’s ticketing authority on United and taking action against the agency to recover the difference in fares by:

· Requiring the traveler to pay the applicable fare differential; or
· Issuing a debit memo to travel agencies for the fare difference and collecting the amount due in full.



Not saying it's being enforced, only stating what they say they will do.