further investigation finds:
"According to The Financial Times, under present laws, a new foreign resident, or a returning Italian who has lived abroad for at least nine years, can pay a flat tax of €200,000 per year on any overseas income and assets for up to 15 years while being completely exempt from inheritance tax on foreign assets during that time."
So it's a voluntary quid pro quo for shielding their foreign assets from inheritance (and income?) tax.