Correction: they chose AGPL with a poison pill (CLA) so they can benefit from it and take it away whenever they want more.
Note that they can't "take it away" completely. The AGPL ensures that the community AGPL licensed code will always have to be open source, and that anybody else can use it too. So a developer who has contributed to an AGPL codebase cannot be denied access to it. Due to the AGPL license, any code will always be open source and thus the CLA cannot claim exclusivity over the contributed code and prevent its (re)use by t…
That's true in the sense that there will always be an AGPL version, but it's not true that "anybody else" can use it to. In many situations you need to acquire a non AGPL license from the original copyright holder to be legally able to use the software in certain environments. That's why CLAs are important so that there is always a copyright holder that can give you a non AGPL version.