Why Voice control not available for iPhone 3G users

Its all in the FW of the phone - there is nothing keeping the 3G from being able to do voice control as you can see from the fact you can download a 3rd party app that does the same thing. There may be some enhancements made possible through the 3GS processor and RAM upgrade, but I cannot say for sure it would exclude the 3G from being able to support native voice control. (Someone else may be able to chime in.)
My guess though...since features sell new models, its not likely we will see this on the 3G from Apple directly.
Then again, anything is possible...
3G iPhone OS 3.0

Yes, and because the law has been very strict about using hands free appliances wile driving, and because that feature has been included in older and more affordable or basic cell phones, that cost less money, I think voice control most be included natively on future updates via software, for a cell phone of the category of the iPhone 3G if possible.
I guess that if you agree, you should suggest this to apple feedback on http://www.apple.com/feedback like I did some minutes ago.
Gateway Pentium IV HT Windows XP iPhone 3G 16GB Black