At the risk of getting modded down again: No, they are not. Pledges are a financial agreement between backers and project creators to exchange X amount of money for whatever reward was promised. Yes, you can back out prior to the ending of the Kickstarter Drive period. That's part of the agreement both parties enter into, but once that period is over that is no longer the case.
From the Terms of Use at
http://www.kickstarter.com/terms-of-use:
"Project Creators are required to fulfill all rewards of their successful fundraising campaigns or refund any Backer whose reward they do not or cannot fulfill."
That's the agreement project creators enter into when they begin their Kickstarter campaign. That's in addition to the common legal protections people have in any agreement made with or without a contract.