The deal for those purchasing the game normally has always been that they would get any new content for a chapter they owned at no extra charge. The timing of WHEN that would happen was never guaranteed, aside from basic decency dictating it would be somewhat reasonable (ie, not months and months of wait). Considering the time involved in developing this new content, and considering the base game only costs $10, I think getting the new content for free, even with a wait, is incredibly generous. It wouldn't be entirely beyond the pale to just charge for it directly, but for a variety of reasons I decided this model made more sense.
You have to understand that this is a business, not a hobby. Everything takes resources, which means everything has to pay for itself, one way or another. A lot of people, especially these days, don't have a lot of spending money, so I thought the best model was to give people a choice: pay with money (via SubscribeStar) or pay with time, by waiting a couple weeks.
From a certain perspective, I can understand the frustration. "I bought the game, why are these people getting the cool new stuff first?!" Because they paid for it. They paid for four months of additional work. And you're getting it for free. The $10 purchase is for the base game, AND the bonus of getting extra content for free, eventually. If you want the new content the moment it's ready, you have to help fund it. I think that's entirely reasonable.