This is probably why its only on the 4S. They probably spent tons on the licence for SIRI from whatever people owned it before, so they have to make a profit on it somehow. If they just distributed it to every iOS device, they would make nothing. They may charge for it down the line, who knows.
I think previous owners of older iPhones would have upgraded anyway. It's not like selling the updated A5 chip and 8mp camera isn't costing additional money for Apple.
Again, selling SIRI for like $19.99 on the app store would more than make up any royalty fees and then some.
The vast majority of people didn't buy the 4S for SIRI. It's a nice feature, but not the selling point, IMO.