People have been copying stuff online for years ("Don't copy that floppy"). However very few people thought "grandmas and 14 year old girls" would be getting online to chat and hangout.
TV stations are oligopoly. They have a business model that works, it works very well (i.e. makes them heaps of money). As long as it continues to make them heaps of money, they don't feel the need to change to meet consumer's demand. In this case consumers want low price (that's a given) and ability to watch programming at their schedule.
Tweeting how much you like "Lost" or liking "Gray's anatomy" episode on Facebook doesn't change the economics, it just delivers more views for their existing business model.
Piracy, on the other hand, delivers what consumers want (ability to watch any premium content on demand, at any time) for a very attractive price ($0). That cuts into network's revenue and that is the only reason they are even contemplating on-line distribution.
They do it very, very begrudgingly, because change is hard and on-line distribution is harder to monetize i.e. they haven't figured out how to make as many heaps of money as with their current business model.
The only reason they do it is that if they ignore this new expectation of their customers, the customers will self serve via piracy, in which case they'll loose even more.
BTW: people don't think of downloading music or movies as stealing (even if you put it in quotes). It is illegal but so is making a mix tape for a loved one or copying an Arnold's hit "Commando" on VHS tape from a neighbor. People have always engaged in some form of copyright infringement. The only thing different today than in 1985 is the scale of those acts which ballooned to a point of causing major drop in sales. The act of downloading a song today is, however, the same as act of a teenager in 1980 copying an album on a tape from a friend. The "stealing" of music of movies has always been a normal and accepted part of life (well, at least as long as there was an technology to do the copying).
Some people are so invested in outmoded business models that they are in denial of this phenomenon and wish more than anything that through some legal or technological trickery it can be made to go away. Others have been more pragmatic and chosen to accept it and to figure out how to live with it.