Why are one way tickets up to three times more than a round trip ticket. And can one buy a roundtrip ticket and never use the returning ticket?
Probably due to several reasons:
1. One-way tickets tend to be refundable and less restricted.
2. More for business travel - as a tourist you are more likely to return back to the place you left from.
3. It stops you from returning on a competitor
Although due to competition from the low-cost carriers some flag carriers (eg BA) offer one-way short-haul flights cheaper than returns but still more than half the price of a return ticket.
If you do not use the returning ticket you would violate the terms and conditions. The airline may ignore it or charge you the difference from what it would have cost for a one-way ticket.