| Nothing burns up. .|
USB port will provide up to100 ma or cut power off. If more current is requested by a USB device, port will provide up to 500 ma or cut power off. It is that simple.
If you do not use the power adaptor, then only 500 ma can come from the computer. That is maybe 100 ma for only four ports. And no power for another three.
With the power adaptor, all seven ports might consume as much as 3500 ma via the adaptor. If any port draws too much, the port simply cuts power off.
Where do the words 'burned' or 'damage' appear in any of this? Doesn't. Did not appear when USB was defined 15 years ago. Can't if one learned basic USB concepts.
Now, if you did not know these numbers, then reread them. Nobody grasps the concepts behind those numbers in a first reading.