I would say a reasonable assumption for someone backing the kickstarter, considering how CR distributes all their other shows, is that they were going to distribute it on their youtube channel or via twitch, which if they hadn't gotten the Amazon deal they almost certainly would have. A not reasonable assumption is that they are going to take the money to make a show and then resell the show to the same people who backed the kickstarter.The Legend of Vox Machina Kickstarter is in an odd place where none of the tiers to back got you the show. It mostly got you merch, or the show script, or such. The most actual show guaranteed was some high tier pledges ($1000+) could get you to specific premieres, but that was just for the show premiere.
Let's just assume Amazon wasn't involved and they were just straight up selling the show on their own. 10 episodes at $10 and they forced everyone, including the backers, to pay to watch it. Regardless of whatever they promised on the kickstarter that would be seen as greedy and underhanded. I don't particularly see this as any different.
The most charitable read I can give it was that they were so excited by the idea of being able to produce more episodes and share more of the story in the show that they made a decision to take a deal that screwed over some kickstarter backers in order to be able to give the expanded show to their community at large (which is what I believe happened). Still not a kinda crappy decision but at least one where they had good intentions.
I will close by saying that I like CR. I like the cast of CR. By everything I have seen of them and every account I have heard they are genuinely good people who are doing their best to spread joy in the world. It's why I believe this decision came from good intentions. That doesn't mean I don't think it was the wrong decision. Even good people can screw up.