The Kickstarter wasn't just selling "the book", it sold four things (we'll get back to this) and wasn't trying to "get back at Amazon". I believe the Kickstarter was an appropriate option, even without considering his inability to independently bankroll the final scope of the project.

The four things:

1. A Premium Hardcover of the novels. This is the first time that the initial print version of one of his novels was released as a Premium Hardcover (albeit they did glue the binding), demand was very much not predictable and using KS helped to ensure everyone who wanted the limited print hardcover could get one (over 90k of each were needed).

2. DRM-free ebook of the novels. This was entirely risk-free for the consumer, they already essentially existed. This was essentially a pre-order, it is really only justified on KS because of #1.

3. Audiobook of the novel. Similar to #2, however I guess there was some minor consumer risk in that the audio needed to be recorded still, but Brandon does have reliable narrators and though he tried and failed at getting special narrators, that wasn't part of the pitch.

4. Swag Boxes. This is the biggest item type to justify KS usage, they needed tools like they get from KS to be able to properly manage the monthly subscription box fulfillment. This did have some consumer risk, because it isn't what they normally do, Sanderson couldn't bank roll it himself (even after the $40M Kickstarter, he's only got a 6M$ net worth) and it was largely an unknown in the book publishing space.

Back to the Amazon bit, it wasn't a selling point to the KS, Amazon isn't mentioned at all. He did decide to support competition in the Audiobook space as part of his fulfillment. In fact, all 4 of the novels are available on Amazon as print and ebooks, published through deals with his traditional publishers.

The way in which he decided to sell these novels (bundled content types and subscriptions) wasn't something his traditional publishers were agreeable to and the KS was used as a proof of concept for that.

The KS raised $41M dollars, it's the largest KS campaign ever by double. There's a 0% chance the project would have been any where near remotely successful (and enjoyed by fans) if he'd tried to deliver it in a more traditional way. He didn't have support of his publishers for that, he couldn't afford it himself and the only other option would be a business loan, which we don't know if he could have received a large enough one. Regardless of funding, the demand smashed expectations, less people would have got what they wanted in a traditional purchase method.

Yeah, there are bad board game companies on KS, take your complaints up with them. Which reminds me, I need to see when my physical edition pledge of Z from 2013 is due...