Even if they wanted to include ethernet, the retina MBP is so thin it wouldn't be able to fit an ethernet port along the edge. Look at this image for reference:

http://assets.sbnation.com/assets/1183987/mbp_comparison.jpg

It barely fits on the normal MBP. The only way it could fit on the edge of the Retina was if they cut through the bevel, which would be ugly.