Understandably, MacRumors posters don't have the best business sense

But that's why I appreciate your post.
Apple sells a lot more iPhones because not only are the expected consumers buying the iPhone 5S, but some price sensitive consumers will now consider the 5C where as they wouldn't have bought the 5S anyway. Apple now has more customers, and therefor more revenue.
The risk they are taking though is brand integrity. If Apple starts dipping into "non-luxury" or "non-product-leadership" phone territory, that makes the phone less exclusive, luxurious, and masterful, which lessons the meaning of owning the iPhone brand. They are starting to muddy up their brand message a bit but they may be able to pull it off with pre-emptive messaging (eg. "un-apologetically plastic"). It's also an attempt of clarifying that their iPhone 5S is masterful, and saying their iPhone 5C competes with Samsung/HTC phones when it comes to customer intimacy (ie., the many colors you can choose and customize to your personality needs).
It's a brand marketing judo move. Subtle, subliminal, and it may work over time. We'll see what they do next year.