This isn't an endorsement of the status quo for consumers, but I do think that publishers such as HBO have the right to sell their content in whatever way makes them the most money - they're running a business after all.
A lot of people seem to assume that HBO and similar publishers either have no interest in or no understanding of new technology, but the fact is that they're as eager to exploit it as anyone else, hence the availability of HBO Go to Amazon customers later this year. That won't have the latest and greatest shows on it, though; not because HBO is trying to make your life worse, but because they have signed multi-year contracs to provide their product exclusively to cable companies for a minimum period. Cable companies want to be able to offer their customers something that they can't get anywhere else in order to retain them as subscribers. Exclusivity or first-refusal deals are the norm in the publishing world, because it's hard to run a business without them.
Again, why do you assume you have the right to be served entertainment through the channel of your choice? You wouldn't call up your local pizzeria and demand they deliver a Big Mac and fries, would you?
Because the marginal cost of a Big Mac and fries is huge compared to the marginal cost of one more person watching Game of Thrones.
Not saying that's a good reason, but that's probably the reason for that assumption. People often fail to consider fixed costs when deciding how high should a "fair" price be.
There is no reason we should sit around worrying about HBO. HBO pays people lots of money to figure out how to monetize their entertainment. Let them figure out how to monetize BitTorrent and similar P2P systems. If they choose instead to ignore those technologies, well, they can suffer the consequences when someone else figures it out. The rest of us should not have to worry -- we do not worry when we turn on the radio or use a VCR, so why should we be so concerned here?