It is is known as a "throwaway segment". It has no value, except as an exchange for another return to complete the round trip requirement of the 1st segment. Almost like a "hidden city" segment, there is no value to the unflown part of a ticket that was purchased with the rule that both parts must be utilized. This is prohibited by fare rule under the CoC. http://content.united.com/ual/asset/COC18May11final.pdf (page 14) "2) THE PURCHASE AND USE OF ROUND-TRIP TICKETS FOR THE PURPOSE OF ONE-WAY TRAVEL ONLY, KNOWN AS “THROWAWAY TICKETING” IS PROHIBITED BY
UA."
For an example, lets say a 1way is $1000, and a rt is $1700. Well, I would be a fool to buy the 1 way if I could buy the RT at a per segment discount and then return the unused part. The difference is more extreme on say a domestic rt segment "excursion" deeply discounted fare where the RT ticket may be less than 1/2 the 1 way. Deeply discounted rt maybe $178, one way is $400.

UA (and most airlines) prohibit "throwaway coupons" they sure as heck aren't going to refund the segment to furthur motivate someone to do it.