I don't believe they are required to make a deal. However, their parent company is a big investor in Uber, so they have some incentive not to drive the Uber into the ground.
This could be the next IBM PC license if Waymo owns the tech that all driverless cars need to operate. They don't need to make much money off Uber, they need to get their tech out there into production so they can refine it before their competitors can get to market.