Animal Club
No one is saying that the practices discussed here are not legal.
Not legal means committing a criminal offence and breach of contract in this instance is not a criminal offence (to the best of my knowledge.)
The example you give is different to the one I quoted, but if you read KLMs t&c's carefully, I think that they could argue their case strongly (since you have in effect changed your itinerary for every ticket) and in any event, they can put you on the refuse list or hurl you out of the FQTV club, since possession is 9/10ths of the law, as the old saying goes.
But since this goes on widely and does not seem to be enforced in Europe at least, then the airlines must be choosing to turn a blind eye... I still wouldn't take the risk, why should I, as a professional I use the ethics argument to justify advice given and if I as busted for flying on a dodgy ticket, it wouldn't make me look too good, would it?