Originally Posted by
cordelli
Because they know it's usually business people who are doing that and there was a time (not so much anymore) they would rather pay the extra to be back the next day then lose another night and have hotel costs, etc for the extra day.
As mentioned, two one ways may cut that cost.
Not only do business people do it to save costs, many business flyer's top priortiy is to get home (regardless of cost) so if they can do it in a day trip they will pay extra.
To the second point, most booking systems (UA's website included) will price as two one-ways if it is cheaper than the roundtrip fare. No need to manually buy seperate tickets.