Hi Chuck! Yes, you can use Strum Machine offline, either through the web browser (offline mode must be enabled with these instructions) or through the mobile apps (where offline support is built-in). All songs are available offline, and you can even create new charts offline.
The subscription model (versus lifetime purchase) makes this “niche” software a sustainable product. I still spend most of my days working to make Strum Machine better for my users – developing new features, keeping up with changes in web browsers and operating systems, responding to user feedback, etc – and I expect that to be the case for many years to come. With a one-time purchase model, I would need to focus on marketing efforts to keep getting new users in the door (and food on my table), and Strum Machine would likely not be nearly as polished a product as it is. With the subscription model, I can focus all my efforts on improving the product… like with the new strumming patterns available for beta testing right now.