The short answer is that FFC needed a dramatic device to set up Sonny's discovery of Carlo's brutality, and the memorable beating of Carlo.
But it was also consistent with Sonny's character: thinking with his dick (as in continuing to play out that comedy with the maid of honor), and his recklessness. A stretch--and I mean a stretch--is that Sonny thought that by picking up Connie, a "civilian," the other gangs wouldn't attack him on his way back to the mall. As I said, a stretch.