Does anyone know how they make money from the free option?
But it's hugely cheaper to wire your house at the same time they are wiring your neighbours. That's the catch -- the deal disappears once the neighbourhood is connected. They're not going to do a truck roll for $300.
Once your house is connected, upsell potential is a lot better than if it isn't.
First, they're hoping many of these users convert to gigabit after a short time ("Just flip the switch, and you get 200x faster internet!"), which will cover the loss from the free users.
Second, straight market penetration. The more people that use the service (even the free one), the more visibility Google gets as an ISP, which has other benefits.