Tweetbot for Mac Alpha is no longer available for download. As Paul Haddad says,
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.
Twitter’s attacks on third-party developers keep getting worse. Still, they haven’t done anything to hurt Flipboard, which I believe would put the biggest amount of attention on this issue due to the fact that it is geared more towards average users and not power users like Tweetbot.