Why I am seeing payment page while downloading free app

You must set up a payment method when creating an iTunes account, even for free apps. The payment method is a proof of identity. After setting it up you have the option of deleting the payment method. See: Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method

Agreed no matter what type of app it is free or not it is going to prompt you for that billing information every time. This is only for the security of the customer.

You can put an iTunes GC on it to finish setting up the account.

Quoting from the link I posted:

If you already have an Apple ID

If you already created your Apple ID, you need to add a payment method when you first use that Apple ID to sign in to the iTunes Store, App Store, or iBooks Store. But you can optionally remove the payment method after you sign in to the store. You won't be asked for a payment method again until you make your first purchase.

Then all you can do is set it up with an iTunes gift card, or abandon the Apple ID you created and create a new one. Again quoting from the article:

If you don't have an Apple ID

Apple recommends using only one Apple ID for all Apple features and services that use Apple ID. If you haven't created your Apple ID, follow these steps to create an Apple ID and store account at the same time, without adding a payment method.

If you're using a Mac or PC

1. Open iTunes, then go to the iTunes Store by clicking the iTunes Store button in the upper-right corner of the window.
2. Scroll down and find the country flag in the lower-right corner of the window. If it's not your flag, click the flag and choose your country or region.
3. Go to the App Store, Music, TV Shows, or Books section of the store.
   
4. Download a free app, song, video, or book. To find free items, look in the right column of the window and click any link that includes the word "Free," such as "Free on iTunes" or "Top Free." When you find a free item, click Free beneath its icon.
   
5. When you're asked to sign in with an Apple ID, click Create Apple ID.
6. Follow the onscreen instructions. When you're asked for payment information, choose None as the payment type.
   
7. After you enter all of the information required, you'll be asked to verify your Apple ID by email. You must verify your Apple ID before you can begin using it. Didn't receive the email?