None of the others are actually transparent, either. They're just white backgrounds. This is usually on white, but in a lot of cases (like in messages), you'll see the white show up against a different background.
A lot of times the images have dark backgrounds because either:
a) the featured item is mostly light or wouldn't look right against a white background
b) it looked cooler if it seemed to be "glowing" (crystals, for instance)
c) it's for variety
d) I was lazy and didn't want to do a lot of photoshop work just to change the background color
The first three are fairly forgivable, the fourth is debatable I guess. Still, over time I'm sure I'll do more that way. So it's mostly intentional, and I'm okay with the way it is. There's a couple, like some of the gloves, where some shading gets cut off sharply, and those I think look a little funny and might get fixed, but the solid blacks are okay.