My understanding is that it's a design feature, although I wonder how often an aircraft of this type would be in a situation where a wing would detach. Perhaps if a wing strikes the ground with sufficient force/angle upon takeoff or landing, often due to wind. Of course, this only makes sense if the aircraft is designed to carry fuel in the wings, which is supposedly more common in more recent aircraft designs. [The downside to such designs is the potential for a rollover, which need not cause fatalities.]