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.
"Luke, I'm yer father, eh. Come over to the dark side, you hoser." -- Dave Thomas, "Strange Brew"