Having read up on it a bit, I don't think the lens designer intended it to be fixed. Almost invariably, when I "correct" the 21mm lens in my raw converter (Capture One), I immediately un-correct it because rectilinear projection (a form of distortion that some insist is "correct") provides an especially unnatural look. It wasn't what you saw - as your eyes are most-certainly not capable of rectilinear projection (we see in a 3D plane essentially).
If you want to use a lens for architecture or shooting resolution charts, then by all means make rectilinear a key criteria. Certainly wouldn't want to shoot a group of people with it. In this shot, the fellow on the right - who is an avid photographer - almost certainly saying to himself "you're going to make me look really stupid if you correct that image" (I didn't):
Sara & James - James Robins - Powered by Phanfare