They do get rendered, they're just really hard to see on that background. Inside a normal post, a horizontal rule has the color #DDD on a background #E3D9D9 (very hard to tell the difference). Inside the preview, the color of the horizontal rule is actually a darker #AAA (for some reason), which has better contrast on the red-grey background. It is there, just not very visible. I'm a tad confused why they're using two different colors for them, though.
Perhaps this will make it easier to see how close together the colors are:
I've drawn a single 1px line through both sides using the other side's color. Can you see it? ^_^