If it becomes commonplace for kickstarted games to offer alpha access cheaply after the kickstarter is over, then that is one less reason to back games on kickstarter and just wait for a pre-order. For the people who do still pledge, alpha and beta access will no longer be of much value, so it isn't much of gesture of appreciation, or an incentive to move to higher level tiers.

During the D:OS kickstarter you could pledge $25 and get the alpha access add-on for $50. If Larian offers the game with alpha access for less than $75, there will most likely be people who complain, with some justification.