Umm, yea you can. You can buy it through the app, no Verizon/Optimim needed. They probably have some clause in their contracts with cable companies that they won't directly sell access, but you can go through Google/Amazon/Apple just fine.
The subscriptions are handled through in-app purchases. I cannot load any of those apps onto my existing devices. So now I have to buy a device, so that I can subscribe to HBO Now?
The point is that the blanket statement, "And you can get HBO without getting cable," is correct but not complete. You still must flow through a provider, and the providers are not universally available. A random person with an internet connection and a computer cannot order HBO Now.
http://help.hbonow.com/app/answers/detailHBO/a_id/172