It's due to the lines of what "Siri" is being too blurred.
If you use the touch screen to set up turn by turn navigation, you're not using voice recognition, but someone's got to read the instructions to you.
Apple doesn't want people to think that Siri works on the iPhone 3GS or 4 because the Siri voice (which varies around the world) is reading some directions in the Maps app.
It's absolutely technically possible to do it, there have been Apps that can do it - even on the original iPhone.
Siri is a collection of somewhat unrelated technologies that Apple brands together in order to make people see them as a unified product/service. To have the navigation feature on other devices would break that.
But if you own Apple products, you have to accept that they drop old products and leave features out purely for marketing reasons.
iOS 4.2 included some new ringtones/alert sounds, but they only worked on the iPhone 4. There's obviously no technical reason for that.