Incidently Firefighters are allowed to do whatever they like in their OWN time. Is that sooo bad.
Yes, it could be, as firefighters should be fit to work at 100% when on duty. They are paid to save lives and property in conditions of risk, operating complex equipment with no margin for error. That means they need proper sleep and rest.
The cash earned in the second job could be a tempation to go short on sleep/rest, IMHO.
Look at it this way, Mikeyd: You'd not want to fly off on your hols with a flight crew that had not been properly rested before reporting for duty, would you?