[sycraft]

there is probably a disclaimer:

Aftermarket Non-GM Parts/Accessories
Many cosmetic-type non-GM aftermarket accessories are approved for Certified Pre-Owned vehicles. In cases where the aftermarket accessory detracts from the appearance, lowers the value, or is not operational, the accessory must be removed (i.e., holes, marks). All accessories must meet approved GM engineering standards (must past inspection) before they can be sold as a Certified Pre-Owned vehicle. If a non-GM accessory is not on the list below and installed on the vehicle, the accessory will have to be removed or the vehicle is not allowed to be certified.
Note: A disclosure must be made on the Bill of Sale to the consumer that non-GM parts and accessories have no GM Warranty. Please refer to 172 Point Inspection & Reconditioning Checklist.