The reason why pax on service 177 can remain on board is because
its route inside MINDEF is not in restricted area. Pax alight at
MINDEF car park (MINDEF pass office) and proceed to enter the
restricted area.
For service 189A, the bus stop at Pasir Panjang Ferry Terminal
(Harbour Dr) is inside PSA restricted area, so only authorised
personnel can remain on board. A service 189A captain told me this.
Same style like services 9, 19 and 89 at CACC.