I think it so they don't have to engage a JW apologist in an endless debate, where the JW just goes round and and round with circular reasoning and refuses to listen to the sound reasoning and clear logic that the 'apostate' is using.
Have you seen some of the comments on these YouTube videos? The JWs are deluded and arrogant, they will never learn and they refuse to take an objective and critical look at their religion. It is frustrating.
So by putting that reminder at the start of the video, all observers can see that the JW who is engaging the 'apostate' in a debate has automatically made himself a hypocrite by disobeying the rules of his own beliefs.