The BMW kit has a little switch box that switches the video feed to the display when you select reverse

The OEM TV tuner provides the following functionality:

Analogue TV reception (useful for eg little wirelesss TV cameras on your trailer)
AV-IN (useful for eg iPod, DVD player, or ATSC digital TV tuner)
AV-OUT (useful for eg driving read headrest monitors)
REV-CAM IN (reverse camera input, useful for... well you know what :-)

Advantage of the TV tuner is that it properly integrates with the nav so that if when watching a movie, the nav needs to display instructions to you, it will automatically switch to the nav video for the duration of the instruction, and then switch back to what it was doing before.

3rd party AV-IN modules don't do that
And I'm not sure if the BMW AV-IN module does that