Not at all! ... but I don't see anywhere I can BUY Siri for the iPhone 4, even though it's technically capable, do you? I would, as would many others, but that doesn't make Apple as much money as buying a new phone would. Apple is happiest if people's options are 1) Buy a new phone or 2) Ess-Tee-Eff-You.
You can't buy it. Apple is trying to tell you the experience would suck more than they want. Again, Apple could have made the iPhone 4S LTE capable but they didn't-- it would suck. Just because you can doesn't mean you should.
Aside from the technical inadequacies of the iPhone 4 to support Siri, I'm sure there are also business decisions, such as making more money for something that cost them a lot to develop. There's a big picture, and some people just don't see it. What this comes down to is the free market and sales numbers. People are free to vote with their money, and are doing so.