Perhaps here on HN. Most people will plug their smartphone into any accepting receptacle. trains, airplanes, NYC SmartLink, or ask the bartender if they can plug it in behind the bar.
I still carry a DIY Altoids charger that takes a 9V battery (pulled down to proper volts for iPhone). In a battery emergency, my phone is simply on life support and I don't have to look for outlets that might also include a zero-day.
I'm with you, this might fall under "safe". Then again, from threads posted here and elsewhere, and through personal investigation...the infotainment systems on airplanes are an absolute disaster with regards to security and software design. They're often part of the same system as the provided USB ports. While the risk is small, there's nothing stopping 1 person from running a script that exploits some flaw in the outdated Linux distro the airline is using to manage their in-flight entertainment.
There's also a chance I'm paranoid and spend too much time here, but I'm gonna stick with my Altoids.
Sometimes I just want to charge my phone from my laptop without triggering all kinds of finder and iTunes and photos interactions.
Same with a car - just power, please.