Well to be clear, it's a ban on the production and sale of foie gras. So you can still import it and eat it and give it away as gifts. You just can't produce it and restaurants/stores can't sell it. There were tons of foie gras parties that happened right before the ban took effect where people just gorged themselves on foie gras.
I think the reasoning is that the method needed to create foie gras is inherently cruel. It's basically force feeding a duck until the liver becomes all big and tasty.