@fembot --
I am not an expert on Open Source or Licensing associated with it, but my understanding is that the creators of Mastodon used the GNU Affero GPL because it specifies that it is intended for software designed to be run over a network, adding a provision requiring that the corresponding source code of modified versions of the software be prominently offered to all users who interact with the software over a network.
The network bit is important considering the purpose of the software