I have a whole bunch of K12 teacher guides and some student pages listed, if anyone is looking. 

It is against K12's copyright and terms of use policies to give these to any student other than the one they were purchased/used for.

How can a company limit a book's use once it is out of their hands?   And it seems, that rule (as quoted by JCC) applies only to the student editions?

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.