As some have alluded to, Siri is really the main feature differentiation for iPhone 4S.
It could be Apple knew this and was very concerned the launch would not get traction so they limited Siri to it. It's a strategic decision. It's not to screw anyone, or "force" people to upgrade. Launches are critical.
Also, the 4S is not really an upgrade path from the 4, it's from the 3GS, earlier, and for non-iPhone users. The upgrade path for iPhone 4 users is the 5 or 6.
Another possibility: Under Sarbanes Oxley (
http://en.wikipedia.org/wiki/SarbanesOxley_Act), it's likely Apple would be required to charge for Siri in an update. They didn't want to charge for iOS5, so they dished that out separately with the intent of selling Siri to iPhone 4 users in the future - and is waiting for the testing period to pass.
There's always little clues in what these guys say on stage and elsewhere - remember, Tim Cook (I think it was) said Siri was
beta - I don't think he was just being cute... I think Siri will come to the 4 soon enough.