From the article:
You can set up Touch ID in iOS Setup Assistant or by tapping Settings > Touch ID & Passcode. To set up Touch ID, you must first set a passcode. Touch ID is designed to minimize the input of your passcode, but you need a passcode for additional security validation, such as enrolling new fingerprints.
So yes, that's how it's designed. You must have a passcode, though as long as you understand the caveats:
In addition, you will need to enter your passcode to unlock your iPhone in these conditions:
After restarting your iPhone 5s
When more than 48 hours have elapsed from the last time you unlocked your iPhone 5s
To enter the Touch ID & Passcode setting
and
You can't use Touch ID for purchases if Require Password in Settings > General > Restrictions is set to Immediately.
you should be able to use Touch ID without needing your passcode. If none of those circumstances is true and you're still being asked for your passcode to make a purchase from the iTunes Store, it would seem that something is amiss with your iPhone.
Regards,