The main reason is really to try to maintain their market segmentation by prohibiting people from buying tickets other than exactly the ones they choose to fly. If they allowed this, it'd be impossible for them to charge more for one-way tickets than round-trip tickets, as many airlines do.
Other prohibited things include: buying a one-stop "A->B->C" ticket and then getting off in B, because you really wanted to go A->B, but it was priced higher; and buying two opposite-direction interlocking round trips, in order to make two short trips appear to be two overlapping longer trips (thus avoiding the "short trip" fare penalties).
Basically airlines are trying to run a purposely inefficient market, and as always happens in such markets, market participants attempt arbitrage strategies, and here the airlines are attempting in turn to prohibit the arbitrage strategies.
I think its even worse. Airlines and their employees know that you have little negotiation power at the terminal (cost, schedule, and security make you virtually powerless).
A story I have is years ago at Continental. I came to the desk to check my suitcase. It was over some weight and so the lady said I had to pay, I believe $75. I had some swag in the bag that I could throwaway that would bring me under. Of course she makes me leave the line to throw it away, although there was a trash can not 15 feet away.
I get back in line, get to the desk and its her again. She weighs my bag... it's under the weight limit. Great! Wait, she still wants $75?! Huh? Apparently I just threw the items away, but the bag was originally weighed more than the limit and her claim was their policy was not to allow reweighings. And certain I've seen people do it before and I ask to see the policy. She says I have to contact customer service, but probably won't make my flight if I do -- and she's probably right on that point.
I'm pretty irate at this point, and ask to see the supervisor and she says she is. She says if I have a problem to take it up with the corporate office, or get out of line. I pay the $75, get on the plane, write a letter to Continental and never hear from them again.
Lucky for me is I hate travelling with anything but hand luggage. So, unless I'm going to be away for a long period of time or theres some other reason why I may need more luggage than I can carry ont he plane with me, this isn't a problem for me.
I personally think flying SFO-LAX is absurd; I drive SFO-LAX, SFO-PDX, SFO-LAS, SFO-SAN, and usually SFO-SEA vs. fly. And I fly Virgin America or Alaska instead of United when possible because I'm very familiar with United's quality of service and product.
No, it's because airline ticket pricing is simply based on the maximum amount of money they can get out of you. Some examples:
A one way ticket nearly always costs more than half the amount of a round trip ticket. So to prevent people from using one half of a round trip, they cancel your entire ticket.
Even better: when Delta had a huge hub here in Cincinnati, they owned something like 80 - 90% of the flights at the airport. They jacked the prices up to the point this was the most expensive destination in the country. However flights out of the Dayton airport 50 miles away were $100s cheaper - including on Delta. And the Delta flight would connect in Cincinnati! When I asked what would happen if I just flew out of Cincinnati and ignored the Dayton - Cincy leg, I was told they would cancel the entire ticket. Same thinking as the first example.
A study at one point showed something like 1,500+ people a day were flying out of Dayton rather than Cincinnati because of the cost difference. While the lost jobs hurt when Delta merged with Northwest and pretty much moved their hub away, I was happy to see them go.
