I don't think Uber's model is necessarily conducive to this, but with small private hire firms it is likely that any case like this would bankrupt them so they may be more careful about drivers they take on. Uber has scale, but that also divorces it from the effects of "one bad apple".
Also, while I agree that taxi drivers are no better than any group - if you've paid for a medallion I doubt you would waste all that on committing a crime like this.
Running an unlicensed cab - now that is a dodgy bunch of people that no-one should get in a car with.
ValueClick Media and Burst Media are a couple of other top tier ad networks to look into. Google AdSense is obviously another option worth considering.
Depending on your niche and traffic though you may be better either directly selling to advertisers (serving/tracking ads through OpenX, Google DFP, or OIO Publisher is pretty straight forward).
Again depending on your niche you might find a vertical ad network - there are these for food, health, religion, music, sport and pretty much any other popular vertical you can think of. They tend to get better ads at better rates - but obviously have higher entry requirements.
(Yes I know the cab isn't owned by Uber, but the analogy fits)