What this says to me is that people really love content with no ads and are willing to pay for it.
Netflix really hits a sweet spot pricing-wise too. I dread the time when they become cable-ified and start adding different tiers (but it would make good economic sense). I've been told Cable TV didn't have ads at the beginning.
Netflix themselves have said many times that this is antithetical to their business model - they want to have a single price for streaming.
They may end up being forced to do this by the cable companies, but this is certainly not their goal if they have their way.