Call centers explicitly pay for being there. That's their job description - being there and being ready to answer calls. That's the whole purpose of call centers - so that you have a phone that you can be sure there's a person waiting on it for your call. That's not what Uber is doing.
It's like I would call an electrician and he'd charge me for a month of work because he sat there for a month being available for my call, it's not his fault I called only now! Would you hire such an electrician? To me it sounds insane.
> As long as Uber have sent the driver a handful of trips in these two hours they'll have to pay no extra.
This makes even less sense. If "being available" has monetary value, as you claim, why this monetary value does not have to be paid regardless of additional monetary value of driving around? If, as you claim, being on call is a separate payable service, it should be charged in the same whatever happened after.
It sounds like this argument is not genuine and is just dragged in to arrive at predetermined conclusion about minimum wage, but as soon as this is not a concern, this argument is discarded completely.