The studio's response (on the linked Kotaku article) is incredibly classy and well written. Completely unoffensive while strictly discouraging this kind of behavior. Kudos.
Textbook social engineering. Look like you belong, and everyone will assume that you do. Easier to just go along your routine than ask about the "new guy".
Slightly off topic: but GTA5 has a mission where you had to do essentially that.
Free advertising for the game studio. Good thing they let him waltz right in.
I want to know if HL3 is coming.
This is why you buy an anti-tailgating system for your unattended but secured doors.