The point was that it's not the entire purpose of having the security guard. The building management wants to "provide security" to its tenants as a feature they are willing to pay for. The tenants, for their part, want "security" but not the inconvenience of being "harrassed" by the guards when they forget their purse or whatever.

Guards know this too, and given the choice between letting someone through who you are pretty sure is legitimate and potentially starting a fight which will generate a complaint, they'll "do the best they can" and just wave people through.

Motivations interact in funny ways, and the parameters aren't like software: they're hidden and surprising.