I always wondered why publishers do that when it comes to PC games ( console games sometimes too).
If a developer doesn't want to work on a PC version, that's probably because they don't expect enough profit from it. So shouldn't giving it to a different studio actually cost more than developing it in house? Fall of Cybertron comes to mind where Mercenary Software is doing the port. And all the Square Enix games that were ported to PC by Nixxes Software ( although those were all decent to great ports).