I didn't know that blowing air into a smoke detector was a warranty issue with the manufacturer, I now know better. I do, however, know legally why blowing air into a smoke detector would be a warranty issue. Here goes.
System is Normal; not System is OK
There are a number of factors involved, none of which by themselves really are important in and of themselves, but together, they make for a problem for the manufacturer.
Starting with "System Normal". The NFPA never talks about the fire alarm system as being "OK", no display on the front of a fire alarm panel ever says "Fire Alarm System is OK", no fire marshal looks for an indication that the fire alarm system is "OK", you are taught to never say the fire alarm system is "OK". There has to be a reason for not saying the system is "OK".
The reason for this is liability. If anyone says that the fire alarm system is "OK" they are implying that everything in the entire system, including anything hidden, is "Totally Functioning and can be Relied on for Everyone's Total Fire Safety and All Property is Protected from Fire".
No one ever knows every detail about a fire alarm system, especially when that person didn't personally install the fire alarm system, or if someone else has made later revisions to the fire alarm system. If anything wasn't designed correctly, if anything wasn't installed correctly, if something hidden has gone wrong with the system, the person that said the system was "OK" can be held liable for all damages that occur from a hidden problem. After a deadly fire, or a fire where there is a lot of property damage, the cause will be investigated, and anything wrong with the fire alarm system is going to be closely scrutinized.
When something that was hidden was not working, anyone that said that the fire alarm system is "OK" is in trouble. That's how liability works.
Smoke detectors are no exception to liability issues.
You know how to clean smoke detectors. You know how to use canned air to clear out the test smoke. Not everyone does. Probably 2% of the fire alarm testing technicians might do something wrong when clearing out the test smoke (I'm not sure what they can do, but nothing is foolproof, someone will find a way to damage a smoke detector).
A larger percentage of building owners can damage smoke detectors when cleaning out dirty detectors, or smoke detectors that they have perceived to be dirty.
Both situations come under the blanket statement of "blowing air into a smoke detector".
Canned Air Doesn't Clean Smoke Detectors
You would think that the dust and dirt inside a smoke detector can be easily removed using canned air, but canned air doesn't work at all on the dust and dirt that is stuck to the side of a smoke chamber. To "Clean" a smoke detector, the blast of air has to be strong enough to dislodge the dust particles - canned air is useless for this.
Canned air is useless because most smoke detectors have barriers that prevent direct blasts of air from reaching the inner walls of smoke chamber. The first barrier is often a screen, which keeps insect out . . . and also disrupts direct blasts of air. The second barrier is the light baffles, which prevent ambient light from reaching the darkened smoke chamber . . . and also disrupts any direct blasts of air that got past the first barrier from getting into the smoke chamber.
Dislodging the dust clinging to the side of the smoke chamber requires a very strong blast of air to get past the barriers. A strong enough blast of air to get past the screen and the light barriers will probably damage something.
It's Everyone or No One
Manufacturers are stuck between a rock and a hard place. In order to sell smoke detectors, they have to say that the smoke detector will detect smoke (they have to imply that the smoke detector is "OK"). But they have to deal with everyone in every circumstance, including those people who don't follow exact procedures and "don't read and follow the instruction manual completely".
On one hand, if the manufacturers say that proper use of canned air will be OK for clearing out test smoke or cleaning out dust, someone will say "but I did take care, the manufacturer did say it was OK. It's their fault that the smoke detector didn't work." Then the manufacturer can be held liable for damages, even though someone else didn't follow exact procedures.
On the other hand, if the manufacturers say that the warranty is void if anyone blows air into the smoke detector at any time, at least no one will hold them responsible for damages if an improperly blown out smoke detector doesn't work.
Fire alarm manufacturers have to be very conscious of liability, or else the manufacturer will be bankrupt very soon. Therefore, the only thing the manufacture will say is "if anyone blows air into the detector, we won't be responsible for what happens." Warranty Void.
To Use Canned Air or Not Use Canned Air
On an addressable fire alarm system, using canned air to clear out residual test smoke doesn't speed up the testing at all; the fire alarm system can be silenced between one detector and the next detector. The next detector activated will just un-silence the panel.
Using the canned smoke according to the directions on the side of the can (including the instructions to not blowing the smoke directly into the side of the detector) will activate the smoke detector into alarm. Using a piece of cardboard, or a magazine, or even a clipboard as a fan to stir up the air around the smoke detector after testing will clear out the detector, and that clearing is without blowing anything directly into the smoke detector. If the smoke detector can't be reset right away, fanning the air around the smoke detector again will usually clear the smoke out of the detector.
For nearly 20 years, this is how I've cleared the test smoke out of detectors. I've done this without using canned air. Instead of canned air, I've used a piece of cardboard as a fan to stir up the air around smoke detectors. Actually, during this time, I never even carried canned air. Fans work.