That's all they need to do. Laws are not about results. They are about convincing voters that they are doing something to make them safer. Besides, if this makes the cost of a gun fractionally higher, a few people here and there will be stripped of the ability to afford a gun, and that's a good thing (as far as they are concerned).
"Am I wrong, or would a criminal only have to pick up his casings to foil this new law?"
You're right, but I don't see a lot of drive-bys coming back to police up their brass.
Most shootings don't happen in the privacy of a home. They happen on the street and people run away. They don't have time to grab the casings. But this is moot because all the guns on the street used in crimes are illegal. OK, 99.9% of them.