More evidence for me :D
Android supports wired headphones with play/pause buttons in principle, but for me, they’ve never worked reliably. Sometimes, I’m listening to a podcast, hit pause, wait a few minutes, hit play, and nothing happens. Android has completely forgotten about the fact that I just listened to a podcast a few minutes ago.
Now Blog1 was written in November 2014 so I would say "current" android and Google probably hasn't fixed much since the blog has written. (Cause come on other than this video not many people seem to focus on this little feature). The unreliability that is described I say gives my theory further evidence.
... well this gives support (I already knew, but didn't explain fully but now gives the reader knowledge) to the feeding signal and how it is manipulated to handle button. That being said I am a little weary on why specifically it is the homescreen ... fudge ... I am second guessing myself now ... well I blame you /u/clb92 ....
.... time passes and I have looked at the Media Playback API ....
So my formal Hypothesis:
The Homescreen falls underneath some module of the whole Android OS that doesn't necessarily listen for the headphone event (what I am calling the pressing play/pause button on the headphones). Whatever is handling the Homescreen/Google Now Launcher has priority in the OS over the most other modules. This priority subdues the 100% active feeding enough such that (given a large enough cable like in the original video) signal can be picked up given the right frequency. However, when the media becomes the priority (or at least a higher one), the media feed begins to be stronger and through calls and checks with the API it causes more signal to be ran on the wire. (It is how your phone knows to play music through the headphones rather than the phone speaker. There are a lot of API calls that do a lot of setting up in that fraction of a second after you launch your media player).
tl;dr
Homescreen does not fully use the headphone wire via the process of API/System Calls thus allowing signal to be picked up like a crystal radio. Once an application is launched then due to API calls the wire is filled with signal originating from the phone and overpowers the Radio signal. (For example, there is a call to see what speaker should be used (SUPER glossing over the actual way of doing it) and checks using electricity through the audio port to see if there is a wire there.)