Actually all airlines (with the exception of jetBlue) bank on volunteers. This is nothing new and shouldn't come as a surprise. Delta offers preferred seating that you can purchase to guarantee you a seat at the time the OLCI starts.

You didn't mention your interview time, but why in the world would you take a redeye flight in for an interview? Multiple things can go wrong very quickly (mechanical, crew availability, aircraft availability, etc) which could have pushed you over to the next day regardless. When I interview folks, I give them up to 2 weeks notice if they have to travel to our location. Part of the interview process is how well you plan accordingly. Booking a redeye or taking the first flight out can cause you to miss your interview, and in the eyes of my bosses, doesn't make for a reliable employee.

Regardless of what the outcome is, I sincerely hope you chose for a check rather than a voucher. File a DOT complaint regardless of the outcome.