Let's face facts. At some point in life, there's a distinct possibility that a robot will take over your job. Once we've accepted that, then we can more easily understand Bill Gates's ideas about mitigating that process by bringing some kind of benefit to mankind. And what better way to do it than to lean on that prime immutable: taxes?
So tax the robot owners or, even better, if the robot is a sentient life form, make HIM... or her... pay income taxes. Of course there's a whole host of problems in making this equitable and reasonable, and then there's the issue of recognizing the AI species and its rights and priveleges. But I think it's a worthwhile trade-off to get the benefit of making robots work for us. Or for themselves. Or whatever.