They get a piece of the 25% in commissions StubHub earns on either end of a season ticket buy and sell transaction. Ticketmaster filed a lawsuit against StubHub and eBay in 2007, alleging "intentional interference" with Ticketmaster's contractual rights.
StubHub receives a percentage of the commission earned from each end of a season ticket sale or purchase, which is part of their business model. Ticketmaster sued StubHub and eBay in 2007 for allegedly interfering with their contractual rights related to season tickets.