They will happily let you pay for it, you just need to have a cable subscription first.

They are going to keep all of their money maker current programming exclusive to their platform for now unfortunately as this is how they currently can afford to make new episodes of Game of Throes for $6 million an episode.

This is a good step though as there is some old content I've been wanting to watch forever which I now can do.

Yeah, you can't. I don't watch cable television, I just use the internet access. So unless I feel like donning a standard cable television package + HBO for $120+/mo I can't get access to HBO Go. It's a shame, I'd certainly be willing to pay for just HBO.