LH is a pretty good example for one way fares responding to the market. About 10 years ago they didn't do cheap one ways at all. When BA started doing the cheap one ways LH responded by offering a similar price structure between Germany and the UK. They don't do it on routes where the other "home carrier" also doesn't do it.

All about maximizing the bottom line. After all, why would you choose to sell a cheap one way, when you can sell an expensive one?