Alitalia was fined for not compensating 2 US bound passengers for cancelled flights. Alitalia paid for cancellations within Europe but denied the two who were flying to the US.
The rules governing an airline are found on the Department of Transportation and other US Government websites; but the individual practices can be different and are only found on the Contract of Carriage of the airline itself. This will be hidden in their website somewhere….
Example: some discount airlines will not put you on a substitute airline if your flight is cancelled; you might get back your one way fare, but you will be hugely out of pocket on buying another one way fare on a non discounter.