You are right but there is quite the selection available all around that could have met thermal and power needs and be more powerful. They could have really gone wild with ATI but I suspect they have a contract with Nvidia.
Its just an issue to me of whats being given GPU wise is well under the power that should be acceptable in 2010 even for non gamers.
They must have a contract with Nvidia, however maybe Nvidia's GPUs really do offer better battery life than ATI's. Apple seems to be pushing battery life over power these days. Nice for the 13", not so much for the larger laptops.