This is a good point. I wonder if there's a market for "power-only" USB cables without data? Or would those fail to negotiate the right wattage?

There's the USB Condom:

https://int3.cc/products/usbcondoms

http://syncstop.com/

While not at guaranteed, killing the data wires (or shorting them) makes the device think it is dealing with a charge only port.

Thus various companies have made devices that conditionally apply said short.

Here is one example: https://www.startech.com/Cables/USB-2.0/USB-Adapters/USB-2-F...

The USB condom is more for situations where you are in public and wish to charge your device from an unknown and possibly hostile USB-A receptacle.

That Startech device still isolates the data line, it just has the added bonus of adding fast charging support. It'd be effective as a "USB condom" alternative.

USB-C has separate lines for the "Configuration Channel", that is used to negotiate voltage and amperage. I'm unaware if the specification allows for cables without data lines, but I don't believe they would be required.

It would only work on USB-C host devices as well though, USB-A would still revert to either 500mA as long as the data lines are tied with a resistor

I have several cables like this. It's always briefly mystifying when I accidentally grab one to sync some data. No negotiation, my fancy devices just charge at 500mA using these cables.