TV by the Numbers has a great article entitled, What Does The Network Of A Failed Show Owe Its Viewers? In short, networks have no obligation.
It's exasperating to see commenters always harping on networks and whining that shows don't get finished. The hard facts are that the purpose of television is to make money and shows go along week by week, not by the finished product. If a show earns enough for network standards, it'll stick around for another year. If it doesn't, it's cancelled.
Sure it would be cool to see every show get proper ending, but what happens in a world when no show gets cancelled in the first few seasons? The network has a bunch of low-rated shows no advertiser wants to touch, and it starts losing money. Soon, the network is no longer financially viable and we either get 10 hours of Leno per week or nothing at all.
Television--especially network television--is not about creative completion and a lasting narrative but money pure and simple. As much as some people would love for the government to produce TV shows and run them forever, that's not going to happen. Powerful businessmen with nice suits will determine what we watch and when, based on how much money they get--and there's not a damn thing we can do about it.
