I have had to point this out to many dealers over the years who try to use those interconnect circuit relays for fire monitoring... If you read the installation sheet for any of the brands (Firex, BRK, etc.) you will find two statements that prevent them from being used (properly) for this purpose.
The first is very explicit - "This device is not to be used for interconnection with an alarm reporting device", or some similar statement. The alarm reporting device would be, of course, an alarm panel. I would imagine there are several reasons it's not deemed reliable by the manufacturers, but technicals aside, from a strictly business perspective, few of us would care to try explaining to an insurance underwriter why it should cover us after a house burns down with the family inside, with no alarm response, because we ignored an explicit manufacturer warning.
The other statement you will find on the install sheet is along the lines of "This device is not to be used in circuits containing smoke detectors with battery backups". Interconnected smoke detectors with battery (9V) backup are designed to reliably operate their detection circuitry, internal sounders and interconnect circuitry while running on battery backup, down to some voltage, as the battery drains. (I don't know what that voltage spec is.) All of these functions they perform very well with a minimum of current draw, to meet certain longevity specs during a loss of AC. It seems obvious part of the design specification did not include powering add-on relays, and to allow the use of said relays on an interconnect circuit that could be running on battery backup when it is needed would nullify the design specification of the system for reliability when operating on battery power. Those relays really only exist for use in special applications not requiring battery backup, such as to activate LEDs in a guard shack or at a nurse's station.
Sadly, I have had more than a couple dealers, when I point this out to them, reply with "Sure, but what percentage of the time will those smokes ever be operating on battery power, anyway? Only during power outages." I always ask them... "In terms of hour-by-hour risk, when is it most likely that system will need to be reliable? When do people start lighting their whole house with candles and oil lamps? When do those who haven't used their wood stove or fireplaces in 10 years crank them up? When do people do stupid things like cook inside on a BBQ grill or with a gas camping stove? When the power is out." As an aside, those same reasons make it even more important not to rely on those relays and expect reliability as more residential smokes are equipped with built-in CO sensing.
It's an easy thing to miss, Ethan. As I said, I have seen many dealers do it. And yes, some of them are even stupid enough to continue the practice after the problems are pointed out to them.
An alternative that is far from perfect, but much more reliable and UL listed for the purpose, is Gentex 9000 series smokes and heats. The (big) downsides - no power supervision of the smokes, no supervision of whether the smoke containing the built-in relay is actually attached to the AC/interconnect pigtail, and no point ID on alarms. Like I said, this is far from perfect - but none of those are any different than what you get with an improperly used relay. The upsides are UL listing, you still have a standard AC, interconnected smoke circuit with 9V backups, and you achieve monitoring from all smokes in the house by taking a fire wire from the alarm panel to a single smoke detector. It also upgrades the house to photoelectric detectors, vs the ionization detectors in most homes. In a surveillance forum it's probably best that I don't launch on all the reasons it is idiotic that nearly all homes in the US have ionization detectors instead of photoelectric.