I believe this was already posted here some months ago:
75% of Linux code now written by paid developers
18% of contributions to the kernel were made without a specific corporate affiliation, suggesting true volunteer efforts. An additional 7% weren't classified. The remainder were from people working for specific companies in roles where developing that code was a major requirement. "75% of the code comes from people paid to do it," Corbet said.
Within that field, Red Hat topped that chart with 12%, followed by Intel with 8%, IBM and Novell with 6% each, and Oracle 3%. Despite the clear commercial rivalry between those players, central kernel development worked well, Corbet noted.
Why should Red hat, Intel, Novell, et. al. care about BSD portability? They are contributing code for their own selfish linux-centric reasons.