This is for a book I'm publishing on CreateSpace. Their support was baffled by my question. This is what they told me:

The interior doesn't have sufficient outside margins for printing, which could result in content being trimmed when printed. The outside, top, and bottom margin requirements are 0.25" for non-bleed interiors or 0.375" for bleed interiors.

THIS is what has me buffaloed. By their standards, that .5" margin would be verboten. They're difficult to deal with, I'll say that...