We could say they choose Ticketmaster indirectly by choosing the venues that have exclusive Ticketmaster deals. Couldn't they pick non Ticketmaster venues if they thought those venues offered them a bad deal?
Between TicketMaster and LiveNation (same company) the number of non-affiliated venues is slim these days. Kikagaku Moyo just did their farewell tour and refused to use any TM venues forcing them to pretty much only play at smaller indie venues.
The reality of the matter is that a artist has much more to worry about than what Ticketmaster handles, and they provide a service that is useful, especially when it can all be rolled up into the "venue costs".
A typical city only has a single venue large enough for a group by Blink-182, Springsteen or Garth Brooks. They are almost universally TM affliated venues, and now with the LiveNation combo promote their own artists on LN-produced tours.
