Originally Posted by rotanes

There are a couple of explanations.

One of them is, that there are a lot of travelers (mainly from government, EU, large corporates etc.), who are forced to fly on the cheapest fares.

Unfortunately on these cheap fares you receive only very few status-miles, so these guys are smart enough to arrange their travel in a way, that they can book high fares (with more status-miles). And the airlines are smart enough to enable these tricks by not offering cheap one-way tickets.

Originally Posted by oxymoron

There are some companies which book these one way fares without caring about the price. So, if on every flight even if lufthansa can get a couple of passengers booking one way fares then they have made their money.