Reputation shield
If artists started charging fair market prices for their tickets the fans would go ballistic and the artists' reputations would be severely damaged. By ostensibly selling the tickets below, even if normal people never get to buy them at face value, they outsource the reputation damage to Ticketmaster. Since Ticketmaster already has a detestable reputation, they are well-equipped to provide this service with minimal damage to their brand. In exchange, Ticketmaster pays a portion of their revenues to the promoters and their associated acts. It's a win win: artists get more money without damaging their credibility with their fans.