The same way as DD free dish is able to operate with profit without having even a single pay channel offered or any NCF charged. You may subscribe to only one channel for 5rs but the additional 250 FTA channels that the operator will give you mandatorily will pay the DTH to carry those channels and without NCF the DTH will definitely get more subscribers and in turn can demand more money from the broadcasters for carrying their FTA channels to give them viewership.
The only place where private operators and DD differs is the maintenance of equipment and technical troubleshooting, for which they can charge the subscriber as and when needed.