You can choose not to connect a Roku TV to the Internet during the initial setup, and you just get access to live TV and the HDMI inputs with (obviously) no streaming channels or updates. It works fine as a dumb TV.
And the credit card thing? That's after you create your account on their website, you can just close the browser window. Or click the button saying "skip" or "later" or whatever it is.
I was once walking past a gay bar with a buddy of mine. A drunk guy stumbled out, looked us both up and down and yelled "Phwoar! That's a porno I wanna watch!".