It makes perfect sense - I'm just not sure how.

The subject line is a direct quote from one of my office mates in reference to the state of the stairwells in this building.  I have seen this in other buildings too, and I found it just as puzzling.  To wit; the exit from the stairwell into the lobby is locked from the stairwell side, but not from the lobby side.  What this means is that while one can ascend the stairs and gain entry to our floor, one cannot leave our floor and gain entry to the building's lobby.  Your only option if you take the stairs down to ground level is to either climb back up to the 4th floor (the first floor with unlocked access to an elevator lobby), or exit onto the street (which triggers a variety of alarms).

Can somebody who knows more about building and fire codes explain the logic of this to me?