The book for my 2010 California says that the socket should not be used unless the engine is not running. Is it not possible to charge my iPhone from here on the go, particularly when using a navigational aid? Just ordered a QuadLock fitting for the handlebars.
Sounds nuts, can’t see why not. 
Maybe it is a pc safety thing like “always wear a helmet”?
Might be because of electrical noise possibly messing up whatever’s plugged in, but then you’ve got 12V lighter sockets in cars to plug anything in, phone chargers sat navs and wotnot and there’s no problems (I’ve got such a socket on the Ural regularly has phone or camera plugged in while I’m riding along, ditto the battery booster [for recharging it]). So it’s probably a disclaimer if you broke something by having it plugged in while the engine is running you ignored the instructions, so not our fault.
In 2010 smart phones were fairly unusual. 10 years later they are everywhere.
I expect technology has overtaken the manual. I’ve even recently added a USB socket on my SP to drive a sat nav, phone charging etc…