If they went that route, they would be limited to 100,000 users. Tapbots likely had more users than that when the new API guidelines were announced, which means that they can have two times their current userbase, a (potentially) significantly higher number, if they keep the same key and secret as for the alpha.
So they have a "limited" beta with the first 10k new test users for each release getting to test it (and once in, you are always in). That would give them 10 testing updates before they hit their cap and a lot of useful data.
