A coworker of mine was putting these on the shelves last week, and complained that it was "lazy" and "cheap" to just have the logo and the character on a white background, and I told him I was pretty sure the objective was an iconic look. Good to know that that's actually quite right. (And I personally think they look pretty good, very stark and direct. No distraction from the main point of the image.)
I think it works as a theme across all of the books. If just one or two books were doing it, then that might fall into the "lazy" category. But the shelves with these books stacked together will probably look really cool, and I'm sure that's what DC was going for.