Based on the ZDNet article linked elsewhere on the comments, this system does not use any tape at all. It is all commodity hardware and hard drives, pretty much in line with the design of the rest of the services from AWS.
Having a multi-hour delay in retrieval lets them move it into their off-peak hours. Since their bandwidth costs are probably calculated off their peak usage, a service that operates entirely in the shadow of that peak has little to no incremental cost to them.
An uneducated guess? Maybe there is some type of spare storage pool just for staging the glaciar restore requests, and they've done the math to figure out how much space they need on average over time for this. The 24 hour storage expiration probably helps with this and they've calculated how much space they need to have on-hand and for spikes for restore requests and the restore delay helps factor in these demand spikes so they can move storage pools around on the backend if they need additional online storage capacity within the next X hours. Plus there could be limited bandwidth to these back-end archival arrays <-> restore pool hosts to save on cost etc which is also part of the pricing equation/delay time.
