The issue is, the union thinks if they stay on as volunteers , the town will never create " Full-Time" Departments and keep using volunteer's.  However, taxpayers with a small community can't pay it.
The issue is most volunteer, then like and go on to a Full-time job somewhere that pays full-timers.
If they have to quit, then should anyone be allowed to work two jobs ?
Firefighters work 42 hours a week, no matter what the schedule, so it is not like they work less.
Anyone can choose to give up their weekend and work more, if they choose to make extra money or just volunteer.
The issue is, would you let a Plumber, Electrician or Police Officer work part-time, in their occupation ?
Actually, they all do work extra, whether for another employer or overtime.  Perhaps, ban overtime, because that takes away another job also. 
If you work overtime, are you not stopping them from hiring another person. Is this not the same point  as double hatters ?