why would a 3rd party head unit cause it and an OEM unit not?
because not all headunits are the same, the alpine ive had before didn't do it, the kenwood nav i just put into my sister's boyfriend's car doesn't as well, actually only all of the f-series, and x-series that i have installed have done it
What is the flaw which causes this particular issue?
let me start off by saying that this is all just a hypothesis, gathered from experience and talking with people much more knowledgeable about this kind of stuff.
this unit is not your average type of radio, it basically is a small computer (it does run off of a windows OS) that connects to an amplifier. i believe that the problem comes from the way that the software side of the unit get its signals to the hardware side of the unit, for instance i have never hooked a computer(with the basic on board sound chip) up to a set of amplified speakers (normal computer speakers that have there own volume control) and it not had the same problem, even when there is nothing playing there is a tiny amount of static that it there all the time, i believe that this is the same thing that is going on here. take kenwood for example, there navs are just a normal radio(a standard piece of audio equipment with different sources(nav being one of them), preamp, and amp with an elaborate display) that uses the nav portion just like any other source, but the avics are a computer(computer that processes every source, a preamp, and an amp) and i dont know if it is just bad engineering or cheap parts being used, or they where just hoping that most people wont hear it(a lot of them haven't until I've pointed it out and let them listen to something that doesn't make the noise)
most of the time this problem is just made worse with cars that have a factory amp, with factory systems that have the amp, every piece of the system was designed to only be used with every part so it works together flawlessly, however with aftermarket equipment, it is designed to be used with all different aftermarket equipment (all made by different companies an not sharing any info with each other so they do the best they can to make it universal) and sometimes it is not fully compatible with factory equipment(as far as the exact type and strength of signals is concerned)