It's not actually illegal to cut spaghetti in Italy.
There's no laws in Italy about breaking or cutting spaghetti in Italy and you won't get into trouble or be arrested for cutting spaghetti in Italy.
However it's also discouraged to cut spaghetti in Italy due to the Italian Culture.
Cutting spaghetti in Italy, while not illegal is considered to be bad etiquette and a culinary faux pas, because in Italy, spaghetti is meant to be cooked and eaten long traditionally.
It's basically viewed as disrespectful to the artisan craft of the pasta maker to cut spaghetti in Italy.
And long pasta is also designed to be twirled around a fork, allowing for better engagement of the sauce and better texture of the spaghetti, which is lost when the spaghetti is broken or cut.
Breaking or cutting the spaghetti in Italy is acceptable though when you add the spaghetti to stews or soups like minestrone, where the smaller and bite sized pieces of spaghetti are preferred.
In Italy, locals frown upon people cutting or breaking spaghetti, although you won't get into trouble doing so.
So if you prefer to cut your spaghetti, when in Italy you can do so without any issues, except you might get some frowns or looks of disgust if someone in Italy sees you doing it.