It's only cheaper because the cost of the space you're taking up on the road and the cost of the traffic you're causing is not accounted for. The solution to this is a VMT tax that accounts for that cost.
Perhaps, San Francisco policy is to use parking prices to discourage vehicle use. They haven't gone the London 'congestion tax' route yet, perhaps they will.
There is a difference between what "is" and how the system responds to what "is". In this case I don't doubt for a moment that drivers of self driving cars would use this technique to economic advantage, and yes if it got too bad it would invoke a response (much like the $5 parking meters with credit card readers in them was a response to cars in general).
