As an aside, I think I know why white objects are often set to Overprint:
No one ever creates a logo or piece of art in Illustrator as White the first time. Usually, the need for a knockout version comes much later after the logo was created and the designer wants to use the logo in reverse. The logo was originally created as Black only, and it was set to Overprint because that's usually what you'd want. So, the designer opens up the Black logo, changes everything to White, does a Save As... and moves on. They don't realize that the logo was set to OP in the first place because overprints are just not on their radar, if they even understand them at all.