When you sign up to be network affiliate, you agree to broadcast almost all of the network content.
There are times you can break that agreement, but they are limited, both in times and hours. If you break that agreement, the network has the right to kick you out as an affiliate.
It's all basic network-affiliate contracts. Now before anyone says, but then a network could be left with a huge deficit of affiliates if there is a mass action by the network against them, in some ways you could be right. But in bigger cities, there are plenty of independent stations who would jump at being an affiliate.