audio - Why can't I use my internal microphone at the same time as my headphones?

I have a Samsung Series 7 NP700-Z5A, which has a dual-purpose mic/headphone jack.

In both Ubuntu 12.04 and Windows 7, whenever I plug in my headphones the internal mic goes completely dead. When the headphones aren't plugged in, the microphone is fine.

The hardware is Realtek HD audio and the driver version 6.0.1.6428 (on Windows, at least).

I have an inkling that it thinks my headphones are a microphone, but there seems to be no option (in either OS) to adjust this.

Why can't I use my laptop's internal microphone while my headphones are plugged in?