Many crews do try to discourage people from doing that, especially at NYP where they're loading food & drinks onto the train. And many crews also frown upon it when they're trying to prep meals. But that said, if the train is not at the platform, they have no choice but to allow passage through that area.
And yes, upon exiting at your destination, they all try to get you to exit to the rear in the hope that you'll tip the attendant at the door.