I was advised that it's because smoke Detectors wired to fire panels are routinely connected in a daisy chain and/or a loop with EOL only after the last device in the series. So when a security panel is connected to an existing set of smokies the practice is continued.
Have heard a few other reasons for it but none were as convincing as that one. Basically it stops teh securitah installer confusing da fire guy.
Some panels will ONLY accept N/O connections to a 24hr fire zone (I'm looking at you, Paradox).
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