Using Android's market share to push or encourage the use of a Waymo robot taxi booking application that is in competition with Uber and Lyft might be seen as anticompetitive.
Other comments elsewhere are "No but if you're Google you can put it on literally every android phone quite quickly." and "All Google would need to do is push a notification to every Android phone that has Google Maps (so, all of them) that says "book your next trip through Google maps and the first ten rides are free!" and they're in business."
That sort of approach would very likely be examined by regulators.
