Not again !!!
It was just a few months ago that we had a extensive debate on 'sequential' and 'complete' use requirements of airlines.
From a legal perspective this has been examined by the UK OFT, and many courts in the EU. Although lower courts have not always agreed with the airlines' perspective to my knowledge it has been ultimately determined that the airline's requirements do not infringe the 'unfair terms in consumer contracts' Regulation.
From the airlines' perspective they have a price for each and every service they provide. They have a prices for one way journeys and for round trip journeys - they even have prices from points beyond their hubs, that are lower than flights from their hubs. The rub comes when the passenger, WITHOUT THE KNOWLEDGE AND AGREEMENT (sorry for shouting) of the airline attempts to substitute one service, which has one price, for another service (which has another price), or a connecting service via their hub with a non-stop service FROM their hub. This is no different that F3G's restaurant customer deciding that he would like the caviar, champaign and lobster thermidor special but at the lower price of the 'daily plat de jour' (and finds some way of fooling the restaurant to give to him at the lower price, without their agreement). A passenger buys a ticket from where s/he is to where s/he wants to go, and usually return to where s/he started - the airlines are simply saying that s/he cannot substitute a different journey (which has a different price) without their knowledge or agreement.
As mentioned above someone who occasionally uses only the outbound without using the return is likely to get away with it - and the airlines are likely to overlook it. But if they detect fraud (and let's be honest, that is what this is) they will do what they can to stop it.
The OP mentioned that LCCs don't do this - and he's right. But the LCC business model says that if the passenger no-shows, they still get the money. But that isn't the case for network airlines. Many of the tickets they offer are at least partially refundable - and if the passenger's ticket was issued by another airline, there is no way for the airline to get the money from the issuing airline.
The real issue here is that the purchaser doesn't like the price being offered for the service s/he wants, and thinks s/he should be able to substitute the price offered for a different service without first getting the agreement of the service provider. Wouldn't we all like to get that First or Business class seat, but only pay for 'back of the bus'?