I mean, there's no doubt in my mind that the reason is Completely Fucking Stupid, but I'm curious what the rationale is anyway.
And further, why, after the decade-plus that we've had graphical web browsers, the software doesn't compensate for this silliness when displaying images? I'm pretty sure that JPEG files can include the gamma with which the images were created, making it possible for it to adjust the image appropriately when rendering it to a different screen, and yet, no software seems to do that.
I noticed this most recently when I walked by rzr_grl's machine and saw her copy of this entry, and thought, "Oh, look at that. I guess bluce has a Mac; here I'd been assuming he just posted a picture that was almost entirely black." But no, it looks fine on her screen.