No but peacefully adding an item to a shopping cart is legal possession under the presumption one will either pay for it or return it to the shelves.
There needs to be further drilling down into definitions. The right to control an item, even if in a constrained manner, may figure in. However it still could lead to some silly consequences. If we were in a bumper car ride and I pushed you out of your bumper car, did I just rob you of the bumper car? Which we both would have to leave at the termination of the ride we had paid a ticket for?