why do I have to pay to download a free app?

Hello~ If you are being asked to add a payment method…then you would need to add one…

Add a payment method to your Apple ID - Apple Support

In most cases there are in App purchases and Subscriptions which need verification. You will not be billed for a free App.

~Katana-San~

Is it asking you to pay a specific dollar amount? Or is it asking you to set up payment information? Even free apps require you to have payment information on file since most of them include in app purchases and upgrades.

You don't have to pay to download free apps at all. You may be seeing one of two scenarios:

• You must have a valid payment method to download apps in the App Store, even if they are free. You will not be charged to download free apps, but you may see a screen to Update your Payment Method if one has not been found.
• If you have a previous purchase that was not able to be billed to your payment method, then you will not be able to download any apps until that transaction has been processed. After that issue has been resolved, you will then be able to continue to use the App Store.