... Not sure where you're coming from on this, Slobodan. It is still typical to frame all art with a white mat around it...
True, however... when printing for matting, one then takes that into account (via white background in LR, for instance, as you rightly pointed out) and makes the whole print a ted lighter.
However, we are here presented with an online rendering on a transmissive screen, and, if clicked, we are then taken to Flickr's pure black background, where the white border clashes with it. A file rendered on a black screen simply glows, opens its shadows and details, appears more saturated.