Lufthansa had a ruling against it in a German "Amtsgericht" court in 2007 for refusing to let a consumer use the return portion of the ticket, without the outbound part.
This is a low ranking court and the ruling doesn't set a precedent.
However, I suspect it is just a matter of time until the consumer rights peeps at the EU cotton on to this practice and take some action.
An airline pricing its routes this way is obviously issuing discriminatory pricing to the disadvantage of some and the advantage of others, not exactly behaviour in line with the concept of a 'common market.'