I suppose the way to look at it isn't that the flights are free, but that you're getting a very good discount on the public rate. Obviously that won't always be the case - you will sometimes be able to get a better deal by booking a public rate - but it tends to be, and especially so in premium cabins.

It's also worth noting that with Reward Flight Saver being rolled out to all routes and cabins except First, you can choose the mixture of points and cash that you'd like to use. If you have lots of points to burn you can spend less in cash, albeit the best value is usually obtained by choosing one of the options with more cash and less points.