Hidden-city ticketing is certainly a breach of contract, and as ExXB says if you are caught doing it you will likely be refused carriage. But there is not much more the airline can do. In theory, the airline could sue you for damages arising from the breach of contract, but clearly this will rarely be a worthwhile undertaking.
It is not fraud; fraud is a criminal offence in most countries - and I am not aware that hidden-city ticketing is illegal anywhere. If it was fraud, in the legal sense, then airlines could quickly stamp out the practice with a few prosecutions, which even if unsuccessful would likely have a dampening effect.
I don't think that breaching the terms of a commercial contract - even deliberately and with premeditation - is fraudulent in and of itself.
But this is by the by; I wonder if Mixture has actually read the article yet?