score:2
Attending a professional workshop while being paid by your American university should fall under the standard short-stay visa.
This is the type of visa issued for tourism, business travel or family visits ; it is also issued so that you can come to France for a short training course, internship, conference, business meeting or be gainfully employed (in whatever way) for less than 3 months. [French government site]
My best guess is that someone thought you were applying for a much longer program.