one could argue that you not paid for that kind of usage and basically pirating in the sense that you use it without permission. It has probably even been denied explicitly to you to break the DRM too. In some countries its even illegal by default to circumvent digital protection measurements.

Just saying, you probably could directly pirate the books too. Putting your money where your rights are respected and you are given the freedom to read your books on any device you want is probably a better idea. Even if it is just to make them grow more.