There's no impossible because if you can see it, it can be captured and digitized, but there is a level of complication that can make it unreasonable. They could make it unreasonable to crack the drm outright and require you to screenshot/OCR it. Then they can limit the OS to make to difficult to automate capture.
Bottom line, they're just kicking payers off their network when it's easier to pirate it than to buy it through their service.