They're not. It appears that they've made a deal (without specific commitments) to sell Uber vehicles that Uber can turn into fully self-driving vehicles if it can. There would still appear to be some reputational risk but Volvo isn't making claims that the cars will be self-driving out of the factory.

autonomous cars will, with time, be safer then the human driven ones, so Volvo may also be playing here their traditional card of an early adopter of a new safety feature.