It depends a lot on whether the pack is intended to carry a bear canister, and if so, what size. If you drop a big cylinder like a Bear Vault 500 right down into a typical pack, that round lump is going to hit the wearer directly in the spine unless there is a serious foam pad in there. If you drop a smaller one like a Bear Vault 450, you might rotate it to put the flat surface against the back panel (axis in the line of travel), so the seriousness of the foam panel can be reduced. If you put a small one like the small Bear Boxer, it is small enough that it can be rotated away from the plane of travel, but still a small foam pad might be good.
If you get up into really huge sizes, then there is probably room inside for so many other clothing things that the foam necessity changes again. The load will be heavy enough that it might be necessary.