I get youtube on my smart TV without any separate sign in. I just click on the app and up it comes 
Most Smart TV YouTube apps will ask you to log in with your google account the first time its used (then remember it). This is same for iPlayer, ITV Hub etc....
Yes I signed in originally to the apps, but that is it. No payment details or anything else but my email address if I remember correctly 
iPlayer etc are free, so no payments attached. Youtube does have an ad-free option via subscription, which if applied to your account, the app should honour. If you installed Amazon Prime video or Netflix etc, again, the app would honour your subscription, and charge your account for rented or purchase options (although all content on Netflix is included in subscription IIRC).
So, like your TV, the fire stick does not ask for any banking details, but does ask to be linked to your Amazon account (it is, after all, an Amazon device). Apps, again like your TV, will need to be logged into in order to work, and you may be charged for using services in the same way.
The NowTV Roku box is the same setup, but needs to be linked to your Sky/Now account, if you don't have one, it will run you through creating an account, which includes payment details (as Nowtv is a subscription service)