Most of his venues have capacity of around 3,000-5,000, whereas The Forum has around 17,000. I remember when this date was announced that someone suggested that they might not use the full auditorium. Anyway, if you're going to paper a show, giving away free tickets with the CD seems a good way to do it.

ETA: There aren't a huge number of seats available through Ticketmaster and they're spread out, so it doesn't look half-sold.