The point is, that to know switching gives a benefit, the candidate must know that Santa will always discard an unpicked, empty, price, and offer you a chance to switch. If Santa barges in, offer you a pick of three, and after your pick he says "well, what if I tell you that this unpicked price is empty, will you switch?", then it may very well be that Santa only offers you a choice if you have initially picked the top price.

Perhaps one should describe the game as:

Santa offers you three parcels. One parcel is filled with chocolate and beer, the others have a goat inside. But from the outside, they all look equal. You get to pick a parcel, but before opening it, regardless which one you picked, Santa will open one of the unpicked parcels, and it will always be a goat. Santa will then always offer you to switch.