Roland Perry
Airlines have attempted to surcharge people who buy a cheaper ticket
than only use part of it (they regard it as fraud). [...]

The key word being 'attempted'. I know people who made 'open jaw'
journeys like that on routes which don't offer sane one-way prices,
but don't know of any successful surcharge. It would be a bit stupid
to penalise one's customers for buying more than they use, as the
seat can be sold again last-minute if there's still demand.