alt Hacker News> Powering something off of any old USB port means it would have to fit within the 5V 500mA basic specification, which the Raspberry Pi 3 exceeded long ago.
In practice most USB wall warts will happily provide 2 amps or even more with no overcurrent protection at all. The 500 mA limitation, at least outside of ports on computers (where you'll maybe get a prompt on Windows and macOS), is theoretical.
Irrelevant...
They need to stick within limits to avoid liability (and avoid arseholes blaming them on forums).
Someone would get a problem with their computer or TV, and then blame Raspberry Pi (or create a problem).