Will Apple Charge my Card without asking me?
Companies can reserve an amount of money on a credit or debit card without actually charging it, to ensure that you can charge the card later when they need to. This is called an Authorization request to your credit card. 

I expect that Apple are reserving a small amount of money when you register a card, and then releasing the reservation immediately, to make sure that the card is valid.

That might reduce your available credit by a small amount for a few days, if your card provider has bad IT systems, but it won't actually charge your account.

This is a standard practice that many companies explain in either their FAQS or T&amp;C's.

From the BBCshop faq.


  I have been charged for my order but it hasn't been despatched
  
  Please be assured that here at the BBC Online Shop we do not charge
  your credit or debit card until your order has been despatched from
  our warehouse. 
  
  We send what is called an authorization request to your credit card
  company. Your credit card company verifies that the card is valid and
  that the amount of your transaction is available. If the card number
  is valid and the funds are available, they put an authorization hold
  on the funds. An authorization is only a hold on the funds for a
  possible future transfer. This hold is temporary, and if the
  transaction isn't settled it will expire without any action. It is not
  a completed transaction and no funds are transferred at that time. 
  
  Your credit card company may still show a temporary authorization hold
  on the funds after your transaction is completed. It can take several
  working days for them to process the void on the authorization. The
  time will vary from one company to the next. Unfortunately, many
  customer service people at the card companies don't understand the
  difference between a settled transaction and an authorization hold.
  They will tell you that it was a valid transaction.


Barns &amp; Noble FAQ.


  Are there any differences in paying with a credit card versus a debit card?
  
  The main difference is that most credit card companies first authorize
  a customer's card when an order is placed, then actually charge the
  account when the item is provided -- in our case, at the time we ship
  your order. The only exception to this is digital purchases such as
  NOOK Books, NOOK Newspapers, NOOK Magazines, and NOOK Apps—for these
  items, the credit card account is charged immediately.
  
  Most debit cards reserve funds from a customer's bank account at the
  time of the original transaction. That amount, therefore, is
  unavailable to you. When the order ships, the bank pays that debited
  amount to BN.com. For digital purchases such as NOOK Books, the order
  amount is immediately debited from the customer's bank account.