They don’t take any money off you unless they reach their goal so if the Kickstarter drive falls short, you lose nothing. Once the drive closes, if the total has met or exceeded the goal, they take the payments off your credit card/Paypal account/whatever at that point. You can change your mind right up to the closing bell.
If it is successful and you pay your money, however, there’s no guarantee that the movie will get made (it may fail for quite legitimate reasons, such as WB changing their minds) but that’s the risk you take.
It helps to think of Kickstarter more in terms of PBS pledge drives than as venture capital - you pledge because you want the thing to happen and/or you want the tote bag/coffee mug/whatever incentive they offer to get you to pledge, but you don’t get a share of the profits if it’s a massive success. All you get is your incentive items and a warm fuzzy glow of making a thing happen that you wanted to have happen.
If you don’t like those terms, don’t pledge. I haven’t regretted my OotS pledge for a moment, but I knew what I was getting when I signed up.