'The third-party doctrine is a United States legal theory that holds that people who voluntarily give information to third parties—such as banks, phone companies, internet service providers (ISPs), and e-mail servers—have "no reasonable expectation of privacy."'
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.
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."
https://www.amazon.com/gp/help/customer/display.html?nodeId=...
Alexa Terms of Use: https://www.amazon.com/gp/help/customer/display.html?nodeId=...
Only Alexa privacy policy I can find links to generic Amazon privacy policy: https://www.amazon.com/gp/help/customer/display.html?nodeId=...
1. http://www.un.org/en/universal-declaration-human-rights/ (Article 12)