[bradfa]

In my experience, USB connected products are EMI tested only with the cable shipped with the product in the box and used per the product instructions or in a "typical" use. Quite a decent amount of effort and cost is spent for some products to ensure that the cable which ships with the product will properly comply with all regulations regarding interference in the countries in which the product is sold.

There's no way a manufacturer could be expected to EMI test a product in every conceivable way a customer might wish to use it. For many products there's simply too many combinations of use-cases or features. So the rules generally require to test in a typical customer use or by following the instructions for use which come with the product.

As soon as you start using cables which didn't come with the product, it's on you to ensure that the emissions of the new cable and the product don't combine to cause problems.

Some cables (not just USB cables) are utter crap for interference.

[Johnythree]

A well designed product will have good shielding and PCB design, and effective low-pass-filters, all of which keep the stray interference within the enclosure. It should not rely on the cable to meet EMC testing, and the test protocol should define a suitable generic cable.

If it does require a particular cable, then the cable should not be removable.

[bradfa]

In an ideal world, I agree with you. But if you ever try to make a mass market USB device you will quickly learn that the computers which your customers will connect your device to and the cables which your customers will use will greatly vary in terms of quality and compliance to the specs.

[Johnythree]

If we had appropriate EMC standards, then this wouldn't be an issue.

Shonky manufactures wouldn't be able to gain advantage by cutting corners.

The standard EMC tests should be specified using generic cables, connectors, etc.