I am not sure if this is what happens, but I could imagine an arrangement where you have a standard, and in order to advertise that you meet the standard, you are required to pay a fee to the standards body, and that fee is used to fund an audit to verify that you adhere to the standard.

It would be nice if, for example, USB did this so that I know a USB cable actually works with a specific standard before I buy it.

USB-IF does in fact do this and has for years. Certified devices are allowed to have a "Certified USB" logo somewhere on them, usually placed next to the regulatory compliance marks.