From their perspective the only way to change the flight is through their documented procedures which requires repricing fares and calculating change fees, even though you are just dropping the return leg. Often roundtrips are less than half the cost of one ways, sometimes even for domestic routes. That being said, I and probably many people on this forum know people who have not flown return segments, or skipped segments to get a lower fare (i.e. EWR > SFO is expensive but EWR > SFO > SMF is cheap so you book the latter and only take the EWR > SFO). While I don't promote such behaviour, there are awkward times where it is really the only option. So long as you only do it once or twice with a given airline they shouldn't bat an eye from what I've heard from others.

Also the airlines have already factored into their intricate models the fact that you won't show up for the flight. They've got decades of historical travel data for them to estimate the proportion of no shows. Hence the reason why airlines rarely oversell their flights.

Safe Travels,

James