Principles behind waza are rarely or not taught in my observation. It is for the student to discover themselves. Even if you teach it this way, the students will not understand if they are not ready to receive. It may also be that the teacher doesn't know how to teach it properly for the students to understand.
Also, the teacher maybe teaching the principles in class, but the student is focusing on something else. It is for the student to discern what the teacher is ACTUALLY teaching. There's always this type of "miscommunication" happening during classes that the gist of the lesson is entirely missed. So you always have to ask yourself "What is he actually teaching?" It's easy to get distracted trying to mimic the technique or criticizing it, like I do myself.
Myself and a number of other teachers have been teaching in that manner for a number of years now. We openly say that the foundation of our teaching is based around the teaching of principles and we explore the end point of the expression of those principles as waza. This experiment in progress has been very positive and beginning students to advanced students all understand this process and benefit from it. My experience has been that it is a more efficient and effective teaching paradigm.