I'd have to disagree. I've been at Amazon for 2 years, and while it's not a sweatshop, it's definitly a challenging environment where I know of no one who works an exact 40h week at all times.
There's zero buffer to slack off, a less productive day is followed by a longer one.
I like working at Amazon. I like the challenge, the projects, the people I work with are fun and smart, but it is not a place where you can easily balance your life for a long period of time. I've never met someone past the 5 year mark that does not define his life as an Amazon employee first, everything second.
You can try to keep yourself at 40h, but eventually it will hurt you, it'll show in your review, it'll hold back your promotion, and if you're unlucky enough to have a management switch at a time where the team is expected to perform, you might even lose your job. I've seen it happen.
Now, maybe in some teams things are different, but in my 2 years, it's been my impression that this is the culture here. You either like it, as I do, I enjoy the rush and the busyness, makes my days fly by. Or you don't and you leave.
I'd be curious to know though, and be honest, you've really ever only worked 40h weeks? You havnt logged in on a weekend or an evening, stayed longer on an Thursday, checked your mails when off work? Not ever? Ignoring on call time offcourse.
As an amazon SDE myself, one who doesn't think working at Amazon is that bad, but do admit that it is challenging, and often do work much longer then 40h a week, I would need to have a guarantee that my pay cut comes with a no more then 30h a week clause.
I'd like to see proper hour counting, like a check in and check out. Where any hour above 30h comes at an extra cost to Amazon, like double pay. So that they would be incentivised to actually tell me to stop working and send me home.
I know some people might say, that's up to you, just don't let yourself work extra, but at a company like Amazon, you can actually lose your job or at least not be promoted from delivering less then the other employees. You're ranked against your peers, so deciding to work only 30h would hurt you in the long run if the others started putting in 35h, 40h, 45h, etc.