Right now they have a gigantic sales force by using the sales staff of all the cable and satellite companies, plus they get to punt on all billing and customer support to the same partner companies. If they were to start selling straight to the customer, hbo would need to staff up their own billing/support, but more importantly, they bite the hand that feeds them. Cable companies wouldn't be pushing nearly as hard to sign people up for HBO if it wasn't going to lock customers in to cable. You'd need a -lot- of sign ups to make up for cable companies switching to promoting the heck out of a different premium channel.
I think the smart move is to milk the gravy train of cable partnerships for a few more years until cable subscriptions fall off more. Then switch to dealing with customers directly. But that's a huge transition to staff up a company and start dealing with end users, and it isn't the risk-free cost-free no brainer that we assume it is when we say HBO should just take our money.