kickstarter is not a store, it's an investment in an idea that might not work
And yet project creators are "responsible for completing the project and fulfilling each reward. Their fundamental obligation to backers is to finish all the work that was promised." Backing a project for a reward that is a tangible item is indeed rather like shopping, with the only real difference being that the product doesn't yet exist (in most cases).
Kickstarter goes on: "If a creator is absolutely unable to complete the project and fulfill rewards, they must make every reasonable effort to find another way of bringing the project to a satisfying conclusion for their backers."
When you buy something from a pukka shop, the deal may still fall through and you won't get what you're promised. The main difference between that and kickstarter is that the deal isn't on until the funding period ends. Between your 'shopping' and the end of funding, enough money may not be raised so nothing happens. In a real shop, you pay and the item is basically already yours regardless of bank loans, other shoppers, etc.
Of course backers are investing. They are investing in the rewards, which have been promised and which should be delivered.
If Kickstarter is an investment, it isn't a very good one, since a contributor puts the whole of their contribution at risk, and in exchange may, at best, get a discounted price on a product they want and can't otherwise get. I actually think this is a good thing, because if it offered the potential of significant upside, it would promote more degenerate gambling behavior amongst contributors.
The text you've omitted between the two quotes you took from the kickstarter FAQ is:
Once a creator has done so, they’ve fulfilled their obligation to their backers. At the same time, backers must understand that Kickstarter is not a store. When you back a project, you’re helping to create something new — not ordering something that already exists. There’s a chance something could happen that prevents the creator from being able to finish the project as promised.
Or putting it another way, this may seem like shopping, its not. I'd suggest a better analogy is buying season tickets (or viewing rights) for a sports team - they may win, they may loose.
Many backers are rallying around their friends' projects. Some are supporting a new effort from someone they've long admired. Some are just inspired by a new idea, while others are motivated to pledge by a project's rewards — a copy of what's being produced, a limited edition, or a custom experience related to the project.
Backing a project is more than just pledging funds to a creator. It's pledging your support to a creative idea that you want to see exist in the world.
Over and over, Kickstarter talks about creativity, creating, creators. Creativity always includes failures along the way. Engineers (and lawyers) put a lot of emphasis on managing risk, but there is also a risk of mediocrity in not taking a big enough risk, and when taking bigger risks, the risk that money will run out before the project is "done."
Kickstarter projects should strive to be realistic in their plans and promises, and finish what they start, but backers should know, that won't always happen, or if it does, it may be late, or fall short of initial promises.