I do agree that more people buying the game is good, and I personally wouldn't be bothered by others who missed the opportunity to join in through the kickstart program paying and being allowed into the beta.
It's not like this is the only game which will have a closed beta. There are a lot of games which are publicly opening up the demos. Usually, they have gone through internal beta testing with large groups (Which I believe was the idea for the kickstarter reward) before undergoing public open beta testing, so they could catch the majority of the problems before it went public. You think the beta for Diablo 3 wasn't thoughly tested internally first? (Well, probably not LOL)
It's not going to be the full game, there is more than likely going to be issues and problems that come up during the beta test period before release. I can understand the devs not wanting people to buy the game on beta thinking it's the release, have bugs and problems and then bad mouth the game and the devs.
(Which quite a few probably would. There are a huge amount of people that don't really understand the role alpha and beta stages of a game are. There are a lot that seem to think that beta tests are nothing more than early access to the completed game, not understanding the part that testing actually takes)
It might not reflect well on the guys and would damage their image, which in turn could prevent us getting more games in the future from them.
All that said, they have said they are going to try and work out something for those that missed Kickstarter, you'll just have to be a little patient and wait and see what they come up with...
Think about it this way, the first to get the beta will not be getting a game that is as polished as those that join later...
Sorry about the wall of waffle, just offering a little perspective from my POV.