Contrary to bizzarre claims made earlier, robots don't actually work for free. In fact, they are more staunch about getting paid than humans are. They will stop working as soon as you stop paying them. Humans, on the other hand, will usually keep working for a while until they finally give in to the realization that you aren't going to pay up.
As a result, we're still going to optimize for the most productive use of robots, just as we do for humans.