bycrew - some airports generally import tourists while other airports tend to export tourists.
Most people flying between Cardiff and Italy are going to be people who live in Wales but fancy a nice holiday in Italy. This means they are spending lots of money (hotel, restaurants, etc) in Rome and the Italian economy benefits. The Cardiff economy by comparison sees very little benefit in return.
Thus if the Welsh Assembly subsidises a Cardiff-Rome route, in effect they are giving a subsidy to encourage Cardiff residents to have a holiday abroad. Even worse, people who can afford a holiday abroad tend to be richer than those who holiday in the UK - thus some of the tax paid by the poor ends up subsidising just the rich - the very opposite of what a fair tax system should do.
If on the other hand, the Welsh Assembly gave zero subsidy but the Italian Govt gave a subsidy it would be absolutely fine. Italian tax money would be used in a way to boost the Italian economy