>which is a model that makes sense for many venues.

This is the point that so many people miss about Ticketmaster. The customer is the venue and not the ticket purchaser. For the venue, there is no one who offers as complete a package of services as Ticketmaster. They provide so much more than the consumer facing ticket sales website that is the only piece of Ticketmaster that most fans see.

I mention the zoning thing only to explain why you can't push "just open a competitor" one level deeper and create your own vertically-integrated competitor - most regions have an essentially fixed number of major venues, so getting 100% buy-in is totally possible.

But yeah, it's not the proximate cause. That's just vertical control of the market, where TM took over online sales for people who needed a POS service for the web, then merged with LN to get control over promotion and venue operation (which among other things means that many box offices charge fees that get passed on to TM).