question

Unfortunately this isn't available. Push notifications is a very popular feature request.

So, the Echo is a bluetooth speaker, right? So any device (e.g. Raspberi PI) could connect to it to emit sound. But the issue is why do I have to come up with a hack to do this?

Some people are already wigged out about having an always-on device listening to everything they say and, potentially, streaming it to the internet. Google got burned when the automatically downloaded the capability to Chrome. Even though they didn't turn it on, some hacker work out how to do it. They lost a lot of consumer confidence when that happened. It's my personal opinion that Amazon will never implement push notifications. (At least without restrictions.) There are too many ways it can be abused that open them up to litigation or create a negative impression with the customers. (Mind you, they certified Ask My Buddy, which is also a lawsuit-waiting-to-happen!) Think about it: If you could push TTS to the Echo, whenever you wanted, and to say whatever you wanted, you could impersonate Alexa. You could say anything from the Echo and it wouldn't necessarily be obvious to the user it was a 3rd party and not Amazon. My "Demotivate Me" does an Alexa impression as an Easter egg. And that's courtesy of the recently added tag, which I also think was not completely thought through. But there you go. I may be proved wrong yet.