This 100w power standard is pretty stupid, though. We're talking power levels where fires will definitely be possible from damaged USB cables.
As opposed to all of the current laptop chargers, AC power cords, DC converter bricks, etc out there now?
Those cables are built for that power, do you want to carry around usb cables that thick for every device that uses usb? Unless there is a way for the chipset to identify the cable (no high power if the cable is a type that's thinner than a certain size) it could be a risk.
You can't trust users to decide sensibly if the thin cable would be safe to charge your laptop if the connector is the same as the thick cable that came with the laptop. Lots of people reason that if it fits, it should work.