CFAA is draconian, the courts are behind the times, and even if you win, it'd be in court where you don't want to be in the first place.

I didn’t check, but the terms of service surely state that you must only enter your own active card numbers for which you are the authorized account holder.

Doing otherwise -> access in excess of authority -> CFAA violation

Just because you can easily walk through an unlocked door doesn’t mean doing so is always legal.