Let me share with you my experience based departure from Samui in April 2016
For what it may be worth, USM (Koh Samui) is privately owned/operated, and NOT by AoT, so suspect they can charge any sorts of fees they think the local market can absorb? Further, any fees would NOT cover those associated with SBIA/BKK.
AoT operate Thailands six international airports: BKK, DMK, CNX, CEI, HKT, HDY.
The DoA/MoT operate provincial airports.
The Royal Thai Navy operates U-Tapao (UTP).