I find this sad and infuriating at the same time. An instructor should be teaching you the aeronautical knowledge you need to be a safe and proficient aviator, not just going along for the flying portion and keeping you from injury while you teach yourself how to fly.
If you're not getting aero knowledge on a regular basis from your instructor, you're doing yourself a disservice.
For all you students out there, the next time the weather is poor, go to the airport anyway, and spend some time on knowledge subjects with your CFI. The airport is a much better classroom than the airplane for those subjects.
It depends on what you contracted for, but in principle I agree completely. Regardless if they teach it to you or not, it's their duty to introduce the material to you to study then review and make sure you learned the material.