Since they appear to plan on running an uber/lyft style service with self-driving tech, and they believe that their tech is especially likely to give them an advantage in that business, that would be one big reason.
the majority of revenue will come from tesla network, it will far eclipse potential revenue from fleet sales, this isn't that controversial what tesla is doing and is what every car company will do.
Revenue from car sales will go down as more and more people use fleet services and car companies will make the majority of revenue from autonomous ride sharing networks, why let some competitor use your cars/software without having to do any of the difficult work themselves, creating a low barrier to entry is a losing strategy if your doing all the work