It's actually more than that. It means that the drivers should be both GPL *and* in the mainline kernel. The second part is often glossed over, but when a driver is included in the kernel the maintenance load on the original author drops significantly. If a vendor upstreams the driver, they don't need to track every single release anymore. One of the major rules of Linux kernel development is that if you change a global kernel API, then you have to change all of its consumers as well. Vendors would get that upkeep for free.
A committee takes root and grows, it flowers, wilts and dies, scattering the seed from which other committees will bloom. -- Parkinson