Big networks even can reserve the right to cancel shows after they air a series they have bought. Remember after Seinfeld ended, the actors who played George, Kramer and Elaine each got their own show. All of them were cancelled after, if memory serves me correctly, the broadcast of only one episode. Later the Elaine actress got a different show which has been successful. The three flops were the year after Seinfeld quit. I don't know the first thing about how the legal end of all this works (what the networks actually bought), but the reality was they decided the shows were bad and not going to be popular, so they cancelled them, based on the audience's response, after airing the premieres. I also know it happens all the time that new series get cancelled in mid-season. Maybe it works different in the cable (particularly the lower budget cable) markets, but for anything big, even seasons are not guaranteed.