You're contradicting yourself here. Many of them don't have fans either.
AppleTV has an internal CPU that generates a lot more heat than a standalone Firewire drive will. But for that matter, you can get external Firewire drives that are better ventilated and have more capacity for the buck.
If you stick a larger drive in the AppleTV, I guarantee you the markup is going to increase the price more than the cost of the same capacity standalone drive.