I think this is a case of technology moving faster than word of mouth advice. It used to be that each cell phone would have some sort of proprietary charging connector and potentially unique requirements. Now, almost all phones and pretty much every single smart phone, charges through a USB port. Anyone who makes a product that has the USB symbol on it (i.e. all companies that make chargers to charge these phones) has to adhere to the USB standard or they're subject to litigation (they can be sued). Part of those standards is the voltage level on the power pins. So if you're using a brand that's at all reputable they'll be putting out a safe voltage.
The phone OEM's can't guarantee that you'll pick a reputable brand, so they say to only use their charger. In practice, however, there are many brands of chargers that will work. There's a lot lower chance of you picking a charger that puts out dangerous voltages on a USB port than there was of you picking out a charger that doesn't fully comply with some proprietary charging requirements that they tried to reverse engineer.