They're probably angling to cut delivery costs by delivering to people's cars while they're at work.
This analysis aligns to what I know about how Amazon rolls out new products and features. There was an A|B test of this, and the option they're rolling out saved a ton of money.
I once saw an interesting research report comparing publicly traded companies based on their outstanding pension / retirement plan debt. The investment thesis was essentially advocating for shorting the companies with the largest under-funding of said plans, based on the assumption that a company that didn't give a crap about its employees probably wouldn't stand the test of time.
I think it's brilliant. If you live in Seattle and know anyone who's a mid-level corporate employee at Amazon, you should notice that it's a great way of selecting talent who are committed to the company and who will stick around. Also, contracts are re-negotiated every two years, so if you chain them up, you're getting a pretty significant annual payout.
Keys and faces don't really matter. It's a 3-part test.
1. You are a government agent
2. You are on a quest for evidence
3. You are searching in a place where there exists a reasonable expectation of privacy
If all 3 of these are true, you need a warrant (at least in the US). Doesn't matter if the keys, or in this case your face, are right there. The fact that you locked your phone with something the cop doesn't normally have is enough to require a warrant.