Unfortunately, only the admin can edit the structure of the library. The positive side is that the structure of your libraries are protected.
For example I worked in a service industry in which all employees had access to our company database to add service requests etc from clients. The drawback was we ended up with single choice lists that became unmanageable. We had a single choice word entered with several different spellings. One ended up scrolling through dozens of choices to get the right one according to his/her preference.
To give you an idea .... we had... Tues, Tuesday, tues, tuesday, Tues. Five different single choice options for one word.
We were forced to limit access to the database structure to only the admins in order for us to provide quick responses to our customer base.
I believe (just my opinion) that this may be why the developer has set up the team subscription this way.
However, if you don't forsee this as a problem perhaps there is
a script available that would allow a team member to add to a single choice list in a library?
Maybe Bill or Ernst could answer that question?