The project itself is almost always also the reward.

e.g., "$50 tier, get a copy of the game"

Unless none of the reward tiers involve the project itself, and are all things like t-shirts and stickers...

I disagree. "5$ donation, get nothing" wouldn't require a refund.

The project purpose is to make a game. That's separate from the reward tier "get a copy of the game". So if the project fails, the backers who bought a copy of said game need to get a refund.

This is some interesting wording. How can a third party enter separate parties into a contract?

Do projects really setup their own contracts with their funders?

Contracts fall under civil law. There is no statutory definition of what constitutes offers and acceptance. So in this case, were I a lawyer, I'd argue that the TOS is a legal contract, and by using the site, you have implicitly agreed to the definition of contract acceptance specified in the TOS.

Whether it holds up in court is a different question, but this lawsuit at least gave a precedent.

Do you have any examples of implicit acceptance of arbitrary contracts merely through accessing a network resource?

Of course not - The contract would be made when money was exchanged, not when the site was accessed.

Sure, but the money is exchanged between kickstarter and the participants (backers and makers). The participants don't exchange directly with each other.

Yup, that is a likely counter argument to be made in court if anyone ever sues over these issues. As I said, though, this is civil law, so it would come down to how lawyers argue it and what judges think. In civil cases, answers are rarely black and white.