A deadlock's purpose is to stop unauthorised entry. Why would anyone want to stop inhabitants from getting out?
If burglars entered via a window, if the doors are deadlocked they are restricted in what they are able to take.
That is what I was thinking anyway.
"In the event of a fire, occupants will be prevented from escaping through double-cylinder locked doors unless the correct key is used. ... The risk can be mitigated by locking the deadlock only when there are no occupants inside the building, or leaving the key near the keyhole."
https://en.wikipedia.org/wiki/Dead_boltBut if the tenants are only supplied with the one key, if it is locked from the outside there can't be a "key near the keyhole".
"Safety
The double cylinder design raises a safety issue.
In the event of a fire, occupants will be prevented from escaping through double-cylinder locked doors unless the correct key is used. This is often an avoidable cause of death in house fires.[4] The risk can be mitigated by locking the deadlock only when there are no occupants inside the building, or leaving the key near the keyhole. Some fire departments suggest putting the key on a small nail or screw near the door at floor level, since the cleanest air is at floor level and you may be crawling to get to the exit, thus placing the key where it is easiest to find.
Note that single cylinder dead locks (with an unlocked twist mechanism on the inside of the door) do not have this problem, and therefore are most commonly used on fire exits. Some areas have fire safety codes that do not allow a locked exit.[5][6]"
"Some areas have fire safety codes that do not allow a locked exit." That might have been what Amaral was commenting on. It may have been illegal to deadlock the doors with the kids inside. Therefore the McCanns may have been more compliant compared to the others of the group who most (if not all) say they deadlocked their children in the rooms while they were at dinner.