As near as I can tell, that's not a "legal opinion", it's simply the court resolution.

Rather than totally ignoring laws, it's far more likely that LCC airline lawyers have determined that "carry on baggage" isn't defined clearly enough in any country's law (or a ECJ ruling), so as to guarantee overhead bin space.

In any event, no LCC is dumb enough to appeal low court decisions such as this one up to a higher level in order to reach a definitive conclusion. They just pay the 50 euros here and there and continue with their business model. If the European Commission decides to make a big deal about it someday, then they will...