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.
I believe the "TX fee" is actually charged to the company that processes the transaction and sends it to Visa/MC. This fee is likely going to be passed on to merchants (and to consumers) in the long run; although the lower potential for fraud will probably offset this added fee somewhat