Paul Haddad explains the rationale behind the move on the company blog:
As some of you may have already noticed the download link for the Tweetbot for Mac alpha no longer works. Twitter’s latest API Changes means now we have a large but finite limit on the number of user tokens we can get for Tweetbot for Mac. We’ve been working with Twitter over the last few days to try to work around this limit for the duration of the beta but have been unable to come up with a solution that was acceptable to them. Because of this we’ve decided its best for us to pull the alpha.
Just to be perfectly clear, Tweetbot for Mac will still be available for sale in the near future, we are just stopping the public part of the alpha/beta testing. We wish we could continue on but we didn’t make the rules, we just have to live within them. Again sorry for the hassle and thank you very much to everyone that has provided valuable feedback during the alpha.
As of this writing, the latest alpha (v0.7.0) still works; I have it running beside Byword’s window.
I love Marco Arment’s take on this. I find myself increasingly disliking Twitter as a company, even though I love their service. The way they want to run their business is fucking up their service.