Generally Medical Only calls shouldn't have a payout (as the payout is for the treatment and transport only).
However, I will look into it.
I think the reason is when the mission was orginally encoded it had a fire pumper requirement & station requirement. Devs removed fire truck requirement (before launch) and still has a fire station requirment today. As seen here: Mission Monday Vehicle and Mission Changes - Errors, Bugs - Mission Chief Board