I think I understand Uber's stance here. From a business perspective their drivers are all independent contractors. This is an important part of the discussion because if they start telling them what the can and cannot do during their time then they start to look more and more like employees (something they absolutely do not want).

An example of this are the popularly discussed drivers that run jewellery or other businesses out of their car while also driving. The moment Uber says you cannot do this on while a contractor for us the legal waters become murkier.

It may be the case that once someone challenges in court an Uber driver (not Uber itself) and wins on the grounds that not accepting a disabled passenger is illegal then the company will have the grounds (and precedent) to enforce that their contractors do not break the law.

To drive the point home, even if Uber wants to prevent their drivers from doing certain things doing so moves them closer to the realm of employees not contractors.

Edit: To clarify I don't necessarily believe that Uber drivers should be considered contractors, but until a case is brought against them and they lose and all drivers become employees they're going to be as hands off as possible to maintain their ability to defend the position. Also providing instructions to a contractor (provide water) does not make them an employee.