Welcomed and encouraged to submit their driver for consideration. I would guess that the vast majority of vendor-proprietary drivers would get rejected from upstream, so for most cases the viable options for vendors wishing have an driver in mainline are to improve an existing reverse-engineered driver or rewrite the driver to fit the kernel standards.
A lot of the drivers are older than their Linux port, which makes mainlining from the start not really make sense. Nvidia and PowerVR I know for a fact are in that category.
