There are too many factors in something like this to make it this basic. If they stole GNU/Linux code then they would be up shits creek big time as it's released under the GPL which has it's own restrictions. If they used BSD code on the other hand, well it's released under the BSD license which has minimal (read: no) restrictions and doing so would be perfectly legal.
This has been discussed to death, so a search. It'll return some interesting reading material at least