Why are one way airline tickets more expensive?

Well, the short answer is because that's what they think the market will bear. Their objective isn't for ticket prices to make sense, but to charge as much as they can for every seat on every flight.

So why do they think they can charge more for a one-way ticket than a round-trip? One-way ticket buyers will often be business travelers (company's paying so why do I care what it costs?) or have some other kind of odd situation going on where they've just got to get somewhere and don't know when they are coming back.

So they think that as a class, one-way ticket buyers will be willing to pay more. Same with charging more without a minimum stay (probably means a business traveler) and many other apparently strange pricing policies.