It's even simpler, I think: you're forcing it to avoid a PD negotiation, and fall back to the lowest common denominator, "500 mA on the PWR line supplied to the USB device". This goes, I believe, as far back as USB 1.1, as a slow-but-generally-safe power source for things that are barely more than "use USB PWR and GND as a dumb 5V source."
A C-to-C cable, OTOH, doesn't have this requirement, and if there's no PD negotiation, the MacBook is not required to provide power IIRC.
- note that all components need to be compliant (macbook, cable, toothbrush)
That's a lot of ifs just to charge a toothbrush. I would be greatly surprised if someone actually did (yes, it might already be cheaper to source SuperSpeed components at scale; I don't yet find it likely though)
A C-to-C cable, OTOH, doesn't have this requirement, and if there's no PD negotiation, the MacBook is not required to provide power IIRC.