Passengers in cars are potentially a huge captive advertising audience.
If that's their end game here, I'd be disappointed.
Going out to the bar? Show the user a drink you know is slightly higher than you know what your user would normally buy.
Coming come from the bar? Show them late night snacking, you can drive them through wherever or they can order right from the interface and it will be delivered to your house.
It could use your spoken conversations to better do search result targeting on a geographical and user level. CGI + TTS can personalize a number of ads just for you "Hey Josh, I saw you bought a domain earlier today, would you like to spin up an instance on Google cloud?". The possibilities are near limitless.
I'm not saying Google wouldn't stumble and fall on something like this. I'm sure in the early days before advertisers craft ads for this medium it will be rough with repetitive ads that are more of an annoyance but advertisers will like having your credit card on file. Soon the advertisers will catch on and provide "rich ads" that allow for ordering/buying/subscribing/+1-ing through the ad (tap on screen to interact or through your phone/tablet/laptop, did I mention? Free WIFI of course, there is even a chromebook you can use if you need).
Maybe it's such that you owe $W for the ride but you can take off X% if you allow Y "permission" or complete Z assignment and there exists enough of Z's / Y's such that the ride can be free. I'm sure it won't be presented that way, it will sneakier, maybe they can even offer users to give them $X if they allow it all. I can see pollers loving be able to target specific areas with questions (and maybe get unique poll results, of course unique impressions does cost more, a lot more). Local businesses running ads during business hours to offer to give users no ads for the rest of the ride if they let the Google car bring them by to buy something, it's on the way to their destination and the next item on their schedule isn't until $X and they have time. Advertise a restaurant and let the user make a reservation right there, don't worry, we put it on your calendar.
There is lot that sounds terrible about that world but I'm conflicted. If it leads to extinction of (over a span of a reasonable number of years for the complete transition) car accidents/deaths (through the removal of both the impaired and just plain bad drivers) due to it's "free" price tag. If it leads to an increased mobility for the lower class now that they have access to transportation. If it push highly fuel efficient or ideally electric cars (even further ideally charged by a solar recharging station). If it does even one of these things then I'm not sure how terrible it would really be...
I'm sure that there will exist, for a time, a netjets-esque "netrides" that either Google or another player will offer that uses nicer/better-kept cars. I'd hope there were online exchanges (or smaller groups running software off github that runs on a Raspberry Pi 3 B :)) where owners of such cars could put their car in a pool of other cars that could be used for rides. You find a way to even out total milage over all cars and users can block out time (daily commutes, special events, vacations, etc). I do worry that Google could eventually choke out these groups due to convenience, cost, and apathy but I have hope. And frankly the car manufacturers only seem to care about driverless cars (no I don't care how long they say some of this has been in development, I'd bet it's underfunded or a joke until recently) now that Google has overtaken them. So while they may all develop driverless cars (or licence it from Google) I'm going to credit a lot of the potential benefits/gains to Google for pushing the envelop and getting them off their asses.
What I'm saying in this extremely long reply (Sorry, I love talking/thinking about this stuff), is that yeah it might be a little sucky but look at the potential gains and the potential alternatives to the "Google car" and see it's not all bad.
On the last flight I took with Delta Airlines they had all the TVs drop down and they played five minutes worth of advertisements on the screens and through the loudspeakers. I just refuse to deal with companies that are hostile to their customers.