Headphones deactivates the proximity sensor?

Perform these steps twice, once without headphones plugged in and once with headphones plugged in:

1) Place phone on a table face up

2) Cover proximity sensor with your hand

3) Initiate a call

4) Remove hand then place back over proximity sensor

In my testing, without headphones plugged in, the screen is active when the call begins and then when you place the hand back over the proximity sensor the screen turns off.

With the headphones plugged in the proximity sensor never activates at all and the screen is always active.

Why does this matter? If you have the phone in your pocket and are wearing headphones, you can start a call using Siri. The screen will activate but will never turn off, meaning that you can potentially hit "end call," "mute," etc., as has sometimes happened in my pocket.

This seems like two bugs to me:

1) The proximity sensor should work regardless of whether headphones are plugged in or not.

2) When a call is started and the proximity sensor detects it is covered, it should keep the screen turned off (it shouldnt require removing the covering then re-covering the sensor).

Any reason why these aren't bugs? Was the sensor designed only for the use case of putting the phone next to your head not for protecting against pocket dials?