There are a lot of users & developers who've been burned in the past by Twitter's moves, one way or another. The problem with requiring cash is that it does very little to encourage people to try on the service without being committed to it.

By moving to this free (with 40 people followed) model, they can allow users to try and thus get more that are likely to upgrade to the higher tier when they want more.

Developers (like @falcon_android) that have hit Twitter's token limit are also getting encouragement to move or support it - as it's not a small subset but a very large one of people who can use the service.