As the article points out, this is not a big cost for the companies involved. Unionisation of the buses is not going to make the slightest bit of difference whatsoever to Google's desire to generate a self-driving vehicle. The market is enormous, so they have all the incentive in the world.
What is going to make a difference to these companies is some degree of collective action. We know that many SV companies have been involved in collection action in the past, with non-compete agreements to keep wages of high-skilled workers low. It is a good thing if the shoe is on the other foot for a while. The only real sad thing is that this is unlikely to spread to where it is really needed -- in the third world sweat shop supply chains.