I have long understood how they operate.
When Common Core was being pushed during Obama's presidency many book companies slapped a Common Core sticker on the front cover. Understanding that different states have different expectations, they also highlight every way that their product addresses each state's standards. Or, how about this one? They rewrite parts of texts, spinning their paragraphs to satisfy the politics of each area.
Don't believe it's true? Check out this article from the New York Times: Two States. Eight Textbooks. Two American Stories. Here is a clear article that explains how the same company markets the same textbook to two states that are very different demographically and politically. In the article there is a lot of evidence to the fact that the texts are reworded to satisfy each audience.
Whether the article is biased or not, it does a very nice job in highlighting the differences in the texts, and it is well worth a read.
It still amazes me that some people believe the textbook is the curriculum. An acquaintance I made at Mount Vernon during the summer of 2018 defines it like this:
Curriculum - A planned sequence of instruction to a view of the school district’s instructional goals. A set of learning goals that outline the intended content, such as state standards, and process goals at specific grade levels throughout K–12.
Instead, we should be curating ideas, facts, and presentations to actually address the school district's curriculum in response to the state's adopted standards.