This is of course a drastic oversimplification.
Modern revenue management systems like United uses don’t have hard and fast “fare rules” with minimum stay requirements like that.
It will always be possible to get a low fare on a route that has a lot of competition and you’re booking on a date that happens to have open capacity (not all too common these days).
Airlines are constantly monitoring what other airlines are charging for the same itineraries. They aren’t going to charge a significantly higher fare unless they are virtually sold out.
They also realize that you could just go on an Online Travel Agent and book 2 one-way tickets. They will never make a roundtrip cost more than the equivalent 2 one-ways as then they’d lose share on their preferred distribution channel.