It might just be accounting practices of the company in question. When I worked at a store (a zillion years ago) and someone wanted a refund it had to be to whatever card it was bought with (or cash=cash). Visa refund to Visa; Amex to Amex; couldn't refund a Visa purchase to Amex, etc. Probably helped balance the books.

Or more probably, this policy helps avoid some potential fraud schemes. Say you bought a tix on your company's Visa, then decided not to take the trip ... maybe thought your boss wouldn't notice or whatever ... hypothetically (if UA didn't insist on refunding the exact card) you could "refund" the tix to your personal Visa. UA and the issuing credit cards would not want to be caught up in that quagmire.

Disclaimer: Not that I've ever done this. I took a lot of criminal justice & forensics in college. This close to getting a job at FINCEN (financial crimes enforcement network) ...