A&C,
Ryanair does not exploit the law as it stands otherwise it wouldn't be under investigation as reported here:
Today in the press - RT News
And this is not a one of a kind investigation, the French have done the same in Marseille.
It all revolves around the definition of " permanent establishment " , it will be interesting to see if Ryanair and alikes will manage to convince Italian judges in this case that around 40 aircraft and thousands of crew permanently based in Italy with plenty of domestic services have nothing to do with Italian laws, social security and taxes......