It is because it is for the Early Access/Beta Testing right? I think you should make that detail more clear because it might give off the wrong impression.
I am not sure if the following is accurate to the state of the game, but I recommend using it if it is.
"The demo is available and free for all, but for now the full game is accessible only to Kickstarter Backers. The 99$ paywall is a temporary block measure that will be lifted once we release the full game for all in a few weeks."