I wonder if the best analogy here is Amazon.

They allow 3rd party sellers in their platform and in their warehouses

There's many reasons for this but they include:

Somebody will compete in every segment: building the taxis, operating them, and vertically integrating them. Put another way - some people will buy taxis in this model either way, so Tesla is incentivized to participate in this also. That helps them get economies of scale.

It also of course minimize risk. But not just the obvious kind. It also minimises risk of a niche competitor taking the buy to own robotaxi market and from that wedge becoming a substantial competitor.

But also like Amazon - operators should worry about Tesla taking the data they have about most successful routes and using that to compete directly in the must lucrative identified markets.