you have been correctly informed. BMW -NA disables the TV while the car is in motion for legal liability reasons. This can be cheated on some versions of the software and hardware, but I don't have the TV/NAV in my E38 so I can't tell you the exact steps. I'd try searching for TV while driving posts to find out what you can do. If you can't use the cheat for your particular TV module, then there is a company that specializes in changing the TV tuner to allow this feature.
www.nav-tv.com
good luck,
John