When a task type (not a custom field) is created, it is added to a ‘library’ and made available to all users. However, I realised that very cleverly, that 'Everyone at [org name] is set as a User, while only the task type creator remains as a ‘Type Admin’. That is a seriously good call, Asana 
I would suggest the same behaviour is applied for when a field is added to the library for consistency .
Currently, when a Field is added to the library, everyone on the organization is set to Field Admin by default. This can have serious consequences if they can all rename the field, delete it or change the options / format etc. Not good, Asana 
PS: The same ‘good’ behaviour can also be seen with Bundles. When you create a Bundle and change the Distribution to shared, Anyone in the Org is also a User
By the way, looking at all these modals together, they seem like they went through different design teams. Can you spot all the differences? 
# consistency
(pinging @Jan-Rienk)