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It for ANY of K12's proprietary content. That's because you have to purchase a course (subscription based) or be enrolled in one of their charter schools to get access to the content. It would be like stealing a transcript of a medical journal or study that you didn't pay for. It's proprietary and cannot be sold, given away, reproduced, etc. They are not typical textbooks. It's content from online.
And it doesn't matter if it's a teacher's guide or a student guide. K12 isn't the only HS'ing provider with these rules, BTW.