Roku is free whether or not you enter a credit card, but many services require a subscription, so Roku thinks it's advantageous to have a credit card already linked to the account. Some might disagree. 🙂
Anyway, you can set up an account at https://my.roku.com/signup/nocc or remove the credit card from an account at anytime. If you decide to link a credit card, you might want to set a PIN on the account at https://my.roku.com so that purchases can't be made without entering it.