How, specifically, do airlines distinguish between people who miss a return international flight for genuine reasons, and people who skip specifically the return leg of a ticket to get a cheap one-way flight on an international flight?
What is the actual legal and/or contract basis of this? Does anyone have any factual experience of it?
For example: when this happens (you genuinely miss or can't take the return leg) does (for example) someone from the airline try to telephone you? Letter, email? Are there any factually known procedures or happenings?
(As opposed to the oft-repeated general idea that the airlines will "go after you" or "try to charge you" in such a case - which is possibly just a myth; hence this question.)
The example fare I had in mind was Europe to the US. In broad general terms this is about 700 euros for round trip but almost always about 1700 for a one-way, so it's a big difference - thousands and thousands of euros for a large family, say - you can imagine the airlines being pissed. {BTW if you're looking for such a fare, a rare exception presently (2015) is XL which offers cheap one-way flights.}