Well I think publishers are aware it doesn't stop pirates.
What I think they hope to do with it is kill the used book market, keep people from giving/loaning books to friends etc. and potentially costing them sales.
They have to know the real pirates won't even be slowed down by DRM, but if they can lose less sales to the second hand market, people borrowing from friends etc., they'll ride DRM as long as they can and use piracy as their excuse/justification as it would be terrible PR to just come out and say they want to kill the used stores etc.
Whether this costs them more money is hard to know, as there's no way to know how much DRM drives people to pirate who aren't people who would have pirated it anyway even if it was sold without DRM.
So we just can't compare money lost from creating pirates vs. money gained by stopping some used sales/borrowing, so we can't tell whether they come out ahead or behind.