Part of it could be privacy too. I ordered a computer monitor that was in its own cardboard box. However, that box had clearly printed "Gaming Monitor" on the box. This item was sitting on my porch for a few hours before I got home.

Amazon put it in one of their boxes, which did not have any labels about the package contents. Thieves are more likely to steal a box that says "expensive computer part" versus an unlabeled box.

Not to say that's always the case, but it depends on what the products box is like.

Better to support returns?

And maybe better protection for items not explicitly boxed to support mail-order/delivery? (Although as noted elsewhere in this discussion, maybe Amazon could use its power to require boxing better suited to their needs from the manufacturers...)

Amazon already (for some items, in the UK at least) makes you explicitly choose to have them re-box it in their own packaging, otherwise they just ship it in whatever box it's in. I imagine they'll do the same here, just not feed these items into the box machine in the first place.