<What is the difference?>
Seriously?  Do you really not understand the difference?
No one says or suggests that the shopkeeper has to stock kosher products.  However, if he does stock them, and is doing business in the public square, he must sell them to any willing buyer, on the same terms and conditions.
Likewise: No one says the baker must sell wedding cakes.  However, if he does sell them, and is doing business in the public square, he must sell them to any willing buyer, on the same terms and conditions.