Because 80 kbps of data is going to be noticeable on someone else's WiFi network.
I really wasn't thinking about phone calls. I was thinking about people using data. That will add up fast if this becomes popular. A lot of people like the idea of a WiFi phone that isn't tied to an expensive data plan. But then you have to see the after effects if these plans take off and people suck up data on other people's dime.
As espaeth said, people already use their smartphones on WiFi when it's available. My Epic 4G probably spends 80% of its life on WiFi, and chews through maybe 70% of its data while on that network. republic isn't going to change that figure much/any.