The AMC network owns AMC and a few artsy channels I've never heard of. Time Warner owns HBO, TBS, CW, TheWB, WarnerBrothers, Kids WB, Cartoon Network, Adult Swim, CNN, and a little more. I've heard that HBO gets $7 per each of it's 29 million subscribers. The parent company, TimeWarner, gets an even larger number per cable television subscriber.
If TimerWarner could sell it's content ala cart and make more money than they currently do off subscriptions I'm sure they'd do it! They have wisely invested in the creation of HBO Go such that if the day comes where that's a more profitable venture, and there are no contractual roadblocks, they'll flip the switch in an instant.
This clarification does not change your point, however. A conglomerated cable channel owner benefits from any one of their channels driving access to all of them. If HBO did circumvent the cable operator, they may be hurting their brothers and sisters in the process
Here's a question- Cable channel AMC makes Mad Men available for purchase on Amazon Instant Video the day after it airs. As far as I know, no cable systems have dropped AMC as a result. Why not?