Himeno

The airlines rely on the relevant authorities telling them if a given area is unsafe to fly in. That generally comes from the authority controlling the specified airspace closing it (as Ukraine did below FL300, or China does randomly) or the authorities where the airline is based telling them not to fly there (as the FAA did for US airlines in Crimea).

Absent any warnings from relevant authorities and given an open air route, the airlines will plot and file the most efficient route available, subject to ATC and ICAO regulations.

Until the incident, there was no reason to believe that the parties of the "war zone" could or would fire on a civil aircraft at cruise.
Cost of fuel does not enter in to it. Either the air route is open, or it isn't. If you must blame someone for civil aircraft overflying a "war zone", it is those who allowed the route to remain open, not the users of that air route.

RandomBaritone

Thanks for being a voice of reason.