Again, there's need to be a distinction between the Project it self and the Rewards. the project funding it self is not an investment nor a normal business transaction, it is a donation and as such it may be acceptable if a project doesn't deliver its final goal.
however, the rewards themselves are not merely a donation! they are as normal as business transaction should be. its a case of paying money to receive a product or a service, may it be a silly t-shirt, an early access to a prototype or a hand printed manual of a game. otherwise, the ones donating $10,000 and the ones donating $5 get the same deal - helping out to fund the project. theoretically, this is ok, i mean, if you have the money and you could donate $10,000, then there is no difference than donating $5, it the cause that counts. but if you promise something else in return for a higher donation, you are forced by any costumer laws to deliver on that promise because it is no longer just a donation, it's a selling of a product (or service).
If you want to look at it in a different way, its very much like donating to some organization or buying cookies from scouts boys going door to door, where the profits of those sells go to the same organization. if you are paying for the cookies, you have every right to receive them, regardless of where or what cause the profits are going to. whether you should morally demand for the cookies is another thing altogether, but it is fair to accept that game development is not as important as other charities and as such, the backer should in no way feel like being a dick by demanding his rightfully bought reward.