Its a small bug. You get the cash although no passenger has this Stop as a destination.
Which payment method (pay_for_total_distance)? IIRC, pftd=0 should pay out at each stop for the distance travelled from the last, so it would not be strange.
All payment methods pay at each stop, which means that you actually may lose money when a vehicle reaches a stop.
If it feels strange that passengers pay even when not getting off the bus, remember that passenger in real life don't pay when getting off either.
A reminder that there's a configurable flag with 3 states within simutrans.conf (if I recall, and there are three possible simutrans.conf locations, one of those being the user's), but it only applies for new maps.