For the vast majority of them, they'll never plug their phone into a non-Apple connector, so the security status of this subsystem will not have any practical importance either way.
I think you underestimate how willing people are to share chargers. If you make one of these malicious chargers, and mock it up to look similar enough to an Apple one, I bet you could compromise a decent number of phones just by hanging out in a popular place (e.g. a coffee shop, or an airport) and making your charger available to folks.
The airport is a perfect example, offering USB ports for iPhone, Android, etc and there would actually be a computer behind the scenes skimming whatever it wanted, or adding whatever it could to the devices connected... A lot of people are eager to plugin to charging stations while waiting for their flights.