Ah, you got me with the old edit while I was replying trick!
Okay, you are saying that you are pretty sure Nero is responsible for adding the icon and believe you have ruled out the other two programs.
You don't have the MPEG2/DVD plug-in, so Nero can't be reencoding the mpeg stream, right? If the other two programs aren't adding the icon while manipulating the mpeg stream, then the icon must be some sort of overlay (I'll admit I'm getting out of my element here). If so, then there must be a simple addition or change to the DVD directory/file structure separate fromt he mpeg stream. Have you done a a file-by-file comparison of the Nero version with the icon to the DVDBurn version without the icon to see where the difference might be?