Translation:Why do you bike without a bicycle helmet?
I'm guessing it's because most of the major languages in Western Europe--I can't really speak for the rest--don't require an article after "without"--thus, English is an exception to the rule.
For instance, with "ohne" (German), "zonder" (Dutch), "sans" (French), and "sin" (Spanish), none of these use an article, just like this Danish sentence here.
Naples is pretty far from a model, especially for urban circulation. Even more so when dealing with the hot-blooded hormone-infused teenagers there. :D
I've experienced it while there. Your day could easily start with a minor hiccup of you dying on an empty stomach on your way to breakfast. Oh, and the honks, the incessant honks dominating space and time!