"It's a part of Android 4.1"

Is it? I think Google Now is part of the "Google Apps for Android" suite. These apps are not part of Android itself, they are licensed by manufacturers to complement their Android-based operating system.

And these Apps are usually updated by Google and independently from the OS. So unless "Google Now" uses a specific API that is only available in 4.1, I would expect older devices receive the app as well.