If there's anyone who knows how to stock up for the holidays, it's Amazon. It's kind of amazing that they sold out on the Echo, all things considered.
Assuming this isn't artificial scarcity (to imply their product is more popular than it is); it just likely means that Amazon discounted harder and further than they projected and thus ran out.
Plus the Google Home had a weak day 1 offering in my opinion and the opinion of many reviewers. So that may have driven sales too.
"If they sold out, the product must be great!"
2) I bet that the average cart size at checkout is less than half of the cost of an Echo.
The benefits don't outweigh the costs of a marketing stunt of claiming that they are sold out.
Birthday is different though, but will not be the case for most decisions.
Long term gains > Short term gains on a product like this.
The main reason I haven't bought an Echo (and am looking into self-hosted solutions instead) is the privacy concern.
I'm also pretty sure that people would've noticed if it always sends by default, guess some people will have monitored traffic from/to the device.