Why must USB low-speed devices use a captive cable?
The reason is, as in your first explanation, to prevent the accidental use of a low-speed cable with a full- or high-speed device.
Yes, you're technically correct that they could simply mandate that low-speed cables be made captive - but having removable cables being used on a low-speed device would create a market for cheap uncertified cables being sold with (or not sold with) an &quot;only for use with low-speed devices&quot; warning that people could ignore and use on other devices.
Making low-speed devices use captive cables simply removes that market and any chance of ever using the wrong cable.