The price they are charging is the bundled price. The fact that you do not get enough value from the bundled price means that you don't have to pay for it, but it also doesn't mean you can steal it.
If you go to the store and someone is selling a 30 pack of toilet paper but you only need 5 rolls because you live alone, you can't simply take 5 rolls, and say, "the price I would have to pay to get 5 rolls is ridiculous, so I'm not paying".
"HBO doesn't offer an option to do that" because they have determined that the revenue/profit they earn from bundling HBO with cable is higher then by selling HBO alone since cable isn't as desirable as it used to be.
The way to think about it is, HBO costs $600/year and you get some crappy free TV with it.
I find plenty of the "free" TV so offensively terrible that I'll nearly pay more for the TV I want just so no subsidization hits the rest.
I like the way Downton Abbey was distributed on Amazon. It was, I think, $2 an episode with a small discount if you paid for the whole season at one. New episodes became available to stream as soon as they aired. I'd happily watch every show I like that way If I could.