An alarm does not always mean there's a fire. Two things are intolerable in a fire detection system. False alarms or a failure to operate in an actual fire. A detection system designed so that a single point of failure can cause an alarm must not be allowed to automatically activate an extinguishing system. At least not where the consequences of inadvertent fire extinguishing agent release are so severe.
IR and UV detectors are notoriously prone to false activation. The way forward in fire detection systems would seem to be a multi-sensor system consisting of different types of detectors that must "agree" before generating an alarm that would automatically activate an extinguishing system.
I guess after being sued for the damages related to false fire extinguishing agent deployment enough times, the manufacturers of such hangar protection systems will spend a little money on designing a more discriminating system. Until then it must be an "acceptable loss".