I have color coded the rejects to correspond to the k bin that is responsible for exceeding $1 \sigma$ and making the tested model non-degenerate with WMAP7. The color coding is actually a spectrum, but one gets the same answer for wide bins in $k$, because it is always one end of $k$ or the other that is causing the model to go out of range. It's pretty clear that this is the reason that the edges of the accepted range are so straight, yielding a parallelogram shape.

Here are a few choices of parameter spaces. The story is the same in each.







The degenerate range does not depend strongly on the range in $k$ that is being probed. The same outside values are the ones going out of range, regardless of what the cutoff value of $k$ is. For example, extending $k$ to larger scales we get:



And, extending $k$ to smaller scales into the non-linear regime we get:


And oh ho ho! What have we here?! It looks like a different $k$ bin is responsible for the rejection, not the absolute outside bin, and furthermore, it looks like the top is a slightly different $k$ bin than the bottom. I will have to look into this more tomorrow!