I think it's the free updates that get WooThemes at least part of their loyalty. While some like the subscription model, many more want to buy a theme and know it will always work. With WooThemes, they can. In fact, they can buy a theme and know that it will keep getting better. That makes a big difference to many.

Then, if they like the theme and keep being impressed at the updates and added features, they're much more likely to recommend WooThemes, buy another theme from them, or upgrade to a subscription model to get all the themes.