I'm a Computer Network Engineer built my first computer in 89. No usb back then but don't knock the power of the usb. I have use my usb ports with everything from atomizers to lava lamps. Usb ports have clean safe power perfect for charging your battery. I have taken apart plenty of computers and I have noticed many are even fuse protected. I suppose it may depend of the manufacturer. What is it about using a usb port that concerns you?
Just a safety thing with me and Li-ion
batteries. I charge all my batteries in another room, on a non-flammable surface, and have a fire extinguisher between me and them. Also if for some reason, not likely but possible, something went wrong, I don't want my laptop screwed up. Like I said, not likely to happen, but erring on the side of caution never hurts IMHO.
Vape On