What would be the legitimate reason to do so? Queries are always preceded by the keyword, so what legitimate reason would there be to record and store audio before the keyword?

I could imagine trying to capture and analyze at least a few words leading up to the trigger word of 'Alexa' to help make it more accurate.

Even if the trigger word is Alexa, different languages, dialects, and slang might vary quite a bit. Some significant portion of people might use "Yo, Alexa...", vs "Hey Alexa...", vs. <a second or two pause> "Alexa...". I'm sure someone with a spouse or child named "Alex", "Alexander", "Alexandra" and so on might also trigger it by accident a bunch, e.g. "Alex, uh, who called?".

Matching all of those trigger patterns would help filter out false positive matches, or take false negatives and refine them so more people can use the product with their own language style and be less frustrated.

You can also configure the wake word as "Echo" or "Amazon" if Alexa is too ambiguous.

"What's the weather going to be like tomorrow, Alexa?"

Allowing that sort of sentence construct violates the privacy afforded by starting with the key phrase. If the Echo did allow this, I would not own one since it would send a lot more unintended sentence fragments to Amazon.

By saying "Alexa" first, it gives me a chance to hear the wake-up tone before I continue my sentence.

So if I'm saying to my friend Alex "Alex uuh, tomorrow we will rob the bank", I have a chance to stop before incriminating myself. But if I say "Tomorrow we'll rob the bank Alex uh... are you in?" and the Echo interprets that as a command, then it's too late, it's already been sent and analyzed.

It's strange that you'd have concerns about privacy when it comes to sentence structure and not that you have a literal black box with a powerful computer and an always on internet connected microphone in it.

Are you talking about my phone, my car, or my TV?

I trust Amazon more than Samsung, Chevy or, well, Samsung. But at least I can verify that my Echo is not sending home unusual amounts of data when it's not in use.

Isn't it really easy to run your own code on it? Not really a black box. Except well, literally.