As a wise man once said:

"If it's stupid but it works, then it's not stupid."

Edit: so these things are called Vespa 150 TAP
https://en.wikipedia.org/wiki/Vespa_150_TAP

TL;DR: it was meant to be parachuted into the field in pairs, one with the bazooka and another with ammunition. The bazooka was actually not meant to be fired from the scooter. You were supposed to take it out, mount it on a tripod, and then fire.