> This may happen once in a while due to some algorithm weirdness, but it is absolutely not "often" the case

It's completely predictable, in fact I checked this claim at Google Flights before posting.

Say you want to fly from LON to NYC, next week, on OneWorld airlines, nonstop. Check the price for a one-way, then check the price when you add a return six months out.

No. It depends on the market, but if it is what's known as a "round-trip" market (typically international itineraries), it shifts to become "often" the case.

Airlines are all about price discrimination. Leisure travellers benefit inordinately from this behavior.

> Leisure travellers benefit inordinately from this behavior.

Is that necessarily true? Charging business travelers more means those businesses have to get the money from somewhere, and one way to do that is to pass the cost onto their customers. So even if you don't fly, you're now paying $0.001 on your next box of cereal to subsidize leisure travelers. Or maybe not! I don't have any data.

It is approximately always the case for international flights.