Surely this is to stop Uber, Lyft etc from buying a fleet of the things for their business use, rather than targeting individual drivers specifically (though they're happy to stop that too).
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