Yeah pretty much all phones have OK Google voice recognition as long as (1) you enable it in Google Settings and (2) the screen is on (can still be locked), or the phone is charging, in which case the screen doesn't need to be on or unlocked.
I always understood that the Moto X has an extra chip/processor that always listens for voice commands with little to no additional battery drain. It may also be a software licensing issue as jackdubl said. I really don't know, but I'm right there with you where I think all new android phones should have this feature.
For now I'm content with it always listening when plugged in, because I only ever really need it when driving, and my phone stays plugged in most of the time in the car.