No reason they couldn't be electric. One of the big barriers for electric cars is a short running length and long recharge cycle. Neither is an issue if your 'car' is just an (autonomous) taxi service that runs off to recharge itself when you're done.

Obviously that doesn't mean it isn't using a fossil fuel somewhere along the power chain, but I imagine the efficiency isn't much worse than bus routes - and it's not like they need to target the exact same use case, either.

Hence my use of the word 'electric' :]