Why do I have to give security code to credit card if purchase is free

You can create an iTune and App Store account without credit card details

1. Sign out of current Apple ID if you are sign-in to one (important)

2. Go to App Store and select a free app

3. Tap INSTALL APP

4. Create New Apple ID

5. Confirm Your Country

6. Agree with Terms and Conditions

7. Fill in your Apple ID and Password (you must create a new Apple ID; don't use your old Apple ID)

8. Create and answer your secret question

9. Select NONE for Payment Method

10. Fill in Billing Address

11. Submit application for new Apple ID

12. Wait for verification email

13. When email arrive, verify your account

14. Start downloading your free apps

Because they expect you to buy something sooner or later.

I have to do this every time I want a free app?