Sometimes smoke detectors fail or get dirty and give false indicators. The damper is in place to prevent the AC system from circulating air during a fire. This will slow the spread of the fire and allow you a little more time to escape. Call an AC company, I'd recommend one that also does residential and light commercial or residential and commercial work, many strictly residential technicians don't have a clue about fire dampers and smoke detectors.
I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.
― Benjamin Franklin