[lolmycat]

From what I understand, the echo has a specific piece of hardware in it that is 'always listening', and once triggered via voice command the echo actually begins to listen. So unless you have something connected to the device that could reproduce that initial voice analysis hardware, you cant have the 'always listening' feature.

[Viper007Bond]

Probably for power reasons, much like the most recent iPhones can be controlled by ; Hey Siri". However older iPhones can always listen too but are required to be plugged in to do it because they're using their main processor and doing it at a software level.

In short, always listening isn't difficult on non-battery devices, it's just a software problem.