Paying bollo is not the main reason. Bollo is around 100-200€ a year.
Third Party Insurance for cars is mandatory and in Naples, due to high level of fraud, it can amount to 1500-2000€ a year.
Using a Polish insurance, or a fake Polish insurance, is the main reason to get a Polish license plate.
Luckily the loop hole is being closed by the Polish authorities after it became popular.
Luckily the loop hole is being closed by the Polish authorities after it became popular.