It is still very early in its development stage so they do not want much liability.
They are selling a product which is a legal and legitimate driver assistance tool which does not have autopilot.
You, as a user, can then modify the device by flashing unregulated code onto it to give it autopilot code, which is not advised by comma.ai *wink wink*
35 million miles is statistically meaningless.
The real point is that Google isn't selling their system to consumers.
Nothin’ like some good old humble startup pie.