Provably fair casinos are not supposed to predict a player's actions. You can ensure the results are random by using a client seed.
That's pretty much become the point of this whole thread:
The issue with that is that there is no user input involved, you're free to always generate a key that results in the right card being the winning one if you check that the new players always pick the left card. The user must always contribute to the process, and it must be done after the server picked whatever seeds it wanted to pick.
