It's not. I think the poster may be confused about how the Echo works - it's not constantly streaming audio or transcripts back to Amazon, just when it's directed to do so via the "Alexa" trigger word.
You are absolutely not consenting to send full transcripts of your conversations to Amazon. If they are in possession of it, it's entirely illegal, and the third party doctrine doesn't apply.
Whether the device constantly streams data or sends it every once in awhile is irrelevant. In the agreement with Amazon you agree to give them them the data, including the voice data. Amazon even lets you manage/delete the data after the fact. They're not collecting it illegally. Whether it is full voice transcripts or not that you're agreeing to send to Amazon is not 100% clear, but that's also irrelevant because by using Alexa you've agreed to send that data. It really depends on Amazon's definition of "when you interact with Alexa." I'm pretty sure that's quite broad.
This isn't Apple fighting encryption backdoors. This is Amazon impeding justice.
"1.3 Voice Services. You control Alexa with your voice. Alexa streams audio to the cloud when you interact with Alexa. Alexa processes and retains your voice input and other information, such as your music playlists and your Alexa to-do and shopping lists, in the cloud to respond to your requests and improve our services."
You are absolutely not consenting to send full transcripts of your conversations to Amazon. If they are in possession of it, it's entirely illegal, and the third party doctrine doesn't apply.