I think the mutual aid agreements here are structured somewhat.  Like size departments provide  mutual aid for routine calls.  More-less, the municipal departments aid the municipal departments and the rural departments respond together to rural calls.  When there is a life at risk situation, anyone who can provide needed aid responds.    Hence, the city department will come to a rural call where someone is trapped, but they won't normally come to a routine call. 

I can see how some might think this is callous and unreasonable, however one has to remember that the department is formed for a specified purpose, and if it is outside of that purpose, it is leaving it's intended clientele without protection.