I'm not a fanboi but normally I don't have any problem with companies expecting you to upgrade to the new model. But claiming Siri is PART OF the 4S when it's actually part of the OS, and deliberately turning it off on older hardware, is massively cynical.
The general public don't realise this of course, but if it does become public knowledge - and it's non-technical enough it could well do - some sort of uproar seems possible. The public like paying for shiny new handsets, but not being duped when the old one is perfectly fine to begin with.