Using the store analogy - when you purchase at a store, the store has a markup on the product (usually 40%) - you cannot demand the store take a different markup because a store down the street is selling it for less, you can't set up a sign in the store telling patrons they can get a better deal down the road, and you can't say you want to pay in a particular currency because you don't have any of the local currency in your wallet. The store can establish policies like this for any patron that enters.
Using the store analogy - when you purchase at a store, the store has a markup on the product (usually 40%) - you cannot demand the store take a different markup because a store down the street is selling it for less, you can't set up a sign in the store telling patrons they can get a better deal down the road, and you can't say you want to pay in a particular currency because you don't have any of the local currency in your wallet. The store can establish policies like this for any patron that enters.