You made a good point when you said a lack of knowledge on the part of students does not mean the history has not been taught. I had not thought about it in quite that way.
What concerns me is when I look at a high school American history text (haven't done that in about seven years) and find only a few pages reserved for the Civil War. As a teacher, you have the opportunity to expand that into as many discussion hours as you wish - if the curriculum planners have allowed you time to do so.
However, just a few pages out of several hundred pages does convey an impression to students of what is and what isn't important, i.e., what is and is not likely to pop up on a test.
Seems like the root problem in this regard must go back to the textbook publishers and curriculum advisors?
I am unsure how the teacher's unions fit into this type of scenario? Maybe you have an idea or two on that also?