Because whilst the studios make the final decisions they also need to keep their customer (retailers) happy. Zavvi fed back how unhappy their customers were with the titleless spines, and a change was made with most recent titles now having spine titles. That's direct feedback from retailer to studio, so yes big thanks to zavvi because they're obviously feeding back the unhappiness from customers at continually preordering based on artwork that is the new changed to something inferior. Whilst they may not be directly responsible for the art they do have an influence, if retailers can't keep their customers happy then the studio loses out on sales, so feedback is a good thing.