I disagree that Apple's goal was lock-in. If it was, they'd probably make the interactive textbooks iOS applications, similar to what PushPopPress did, instead of this bastardized version of epub.

Quotes from the linked post like this: "The ability to control the size of each column and column gap was recently discussed in the CSS WG. The Group decided that allowing setting of individual column width and column gap width is not a feature considered for the first REC of this document.", suggest, to me at least, that Apple created their proprietary format because standards didn't and still don't cover the fictionality they have in mind for these interactive books. I'd rather Apple do that than hastily muscle their implementation into the standards.