Although it's against the rules, airlines rarely enforce the rule and I know of quite some examples where my colleagues have also not used the return flights in order to cut costs for their departmental travel budget.

It does lead to no-shows for the airline if you do not cancel and that in turn makes airlines overbook their flights again.
But I think these full fare return fare policies are really old fashioned pricing mechanisms.
One should be able to book a oneway at a reasonable price and until this is in place, people will always try ways to circumvent the airlines policies.