That has not been my experience at all. I just checked a flight from Gothenburg to Amsterdam leaving in three weeks time(a random example), and a one-way ticket was almost exactly half the price of a return flight. In fact it's been a long time since I've seen too significant a discount for a return flight vs two single flights.

For the cheapo European carriers, you are generally billed for each leg seperately, and there is no discount for buying a return ticket.

But the real reason for looking for a one-way ticket was simply an attempt to speed up the awful search times.