USB charging, part 1: requirements

No, it means that the host *may* refuse to supply sufficient charging current.

There are lots of USB-powered thingies around, like fans for summer, hand-warming mittens for winter, and pretty blinking lights for the festive season. They don't enumerate the bus, but they work fine when plugged into most USB sockets. If you tried to plug 4 of them into an unpowered USB hub, I suspect they wouldn't work. Hopefully the hub would detect the excess current draw and turn off.

The key point about battery powered mobile devices is that sometimes you plug in a USB cable to charge, sometimes to transfer data, sometimes for both. The device needs to know what to do. If it just pulled all the current it could, like the warming mittens, it might cause the port to shut down and then you would never get your cat photos. The mittens have no cat photos, so causing a port shutdown isn't a concern.