Some people are saying that turn by turn does work on iPhone 4 in the beta but this is a misunderstanding on what turn by turn means. It does update the directions as you go along on iPhone 4 but it does not give you audible directions so basically just like the current google maps based one but auto updating. The reason for no turn by turn is to help differentiate between the 4 and 4S, i.e. a reason to buy a more expensive 4S over the 4. Same reason probably applies to Fly over.
I can only confirm that I read the same. Also, "transit" has been completely removed from maps for all devices... bummer.
Why does the 4 not get flyover? Apple will probably claim hardware limitations. Every time they make that claim the jailbreak community has proven that to be BS. It's more of "we want you to buy our current/newer devices".
All that said, this is all beta still. A lot can change before it goes gold.
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.