I guess you need to have money on your apple pay account or to have it connected with credit card and bitcoin comes in as a means of adding funds into apple pay account.
You don't actually have a "balance" on your apple pay account. When you use apple pay you are essentially doing the same thing as swiping your credit card, except it is somewhat more secure as your card number is encrypted the entire time from your phone until it reaches visa
You are actually paying apple to swipe your credit card.
Apple levies transaction fees on top of the charges levied by the credit companies.