Intel iMac fails to isolate audio to left or right speaker on channel 1 and 2

I have noticed that if I have a stereo sound source that sends audio on only one of the two channels (left or right), my iMac will send that sound to both channels so that I hear it through both speakers. This is undesirable behavior.

Here's how I can reproduce the problem: In Audio MIDI setup, under Audio Devices, I select an external audio interface (Audient iD14), and click Configure Speakers. Then a popup appears that allows the left and right speakers to be connected to channels on the interface. In the case of my iD14 interface, channels 1 and 2 are selected by default. When I click to test the left speaker or the right speaker, I hear white noise from both speakers. This unexpected behavior is the problem. I would expect sound to come from only the speaker I am testing. 

However, if I connect the left and right outputs in Audio MIDI Setup to channels 3 and 4 on my iD14 interface, then when I click the right speaker, white noise emanates from my right speaker only, and when I click the left speaker, white noise emanates from only the left speaker. I consider this correct behavior and this is how I would expect channels 1 and 2 to behave as well.

I can reproduce this problem using the internal sound card in the iMac, so it appears to be Apple's bug. 

This is my test with the internal Apple sound card: In Audio MIDI setup, under Audio Devices, I select iMac Speakers. Then I click the Configure Speakers button. This opens a popup that depicts the left and right speakers. By default, the left speaker is on channel 1 and the right speaker is on channel 2. When I click the left speaker test button, I hear white noise from both speakers. I would expect the white noise to come from only the speaker I clicked. If I click the right speaker test button, I get the same problematic result -- white noise from both speakers.