i found this
i found it at yahoo answers
Round-trip tickets are often discounted. One-way tickets are almost never discounted. Airlines price tickets to get business travelers (who have to fly at very specific times and are willing to pay for the privilege) to pay more, and discount tickets for pleasure travelers (most of whom are willing to fly on different dates or even to different destinations to get a better deal – so you can’t make them pay much more or they’ll go elsewhere). That’s why they usually require advance purchases and Saturday night stays for a discount round trip. If they had discount one-ways, that would let people get around all of the restrictions on round trips, and would defeat the airlines’ whole pricing model designed to ensure that business travelers pay more and only pleasure travelers get the discounts.