Agreed, although there are USB "charge" cables and USB "data" cables. The charge cables presumably have no data path. I discovered this when unable to download from a d!git@l camera through one. I then ended up going through a bunch of my accumulated cables and marking them appropriately.
I do usually use a wall-wart, mostly because a hefty one charges faster if it's compatible with the ever growing "features" of USB devices.
Typically those and the laptops I've had were the 50-60 hz, 100 - 240v goodies, so a plug adapter was all that was needed for other parts of the world.