I think that's more of an issue with 'hidden city ticketing'/deliberately missing connections, because it's cheaper than buying a ticket to the layover location.
But if it’s just A-B and B-A and you don’t take the return flight, no one cares. (I’ve even gotten flack for notifying the airline as a courtesy…)
E.g. LAX-NRT on 3/2 on UA 32 is $966 right now. Add a return flight on 3/9 and the flight _drops_ to $893.
I'd be surprised any airline gave you flak for telling them you were missing a segment. They might try to charge you more (!).
They do indeed, including trip change fees. For myself, I luckily got an agent who was nice enough to ignore what I had told her.