Cults have a hierarchy for a reason - if you put someone new or someone that's less indoctrinated/invested in a position that exposes the cult in some way then it's more likely that they're going to wake up from it. I think the cart witnessing is like that - it was always fairly apparent to me as a lowly publisher that we didn't use methods of preaching that were likely to be very productive, and I'm sure I'm not alone. Just imagine what a publisher that already thinks that a little would think after a day of watching people pass by him. This could be enough to wake someone up a little, or at least send them on a path of inquiry.
Also, it seems that they intend to trumpet the success of the cart witnessing at conventions and such, so it's best of if only the super indoctrinated are aware of the fact that these experiences are either fabricated or by far the exception.
Plus, as others have said, it's a reward for the pioneers and might make some strive to pioneer thinking that it will be easier now because they can just stand by a cart. The pioneers are more likely to just be thankful for the easy time and not ask questions.