sorry that's the way it works. they don't charge you anything but keep your info on file in case you buy anything through the stick.
you can subscribe to a ton of services and buy/rent movies and tv shows through it as well. this way you're not entering your info for each app/service you want to use. roku and apple tv do the same thing.
if you really want to, give them a cc number for an account you don't use. roku has an old debit card for a closed account on file for me