Can't speak for every person at AWS, but I can say that I'm personally happy with my work/life balance working at Amazon. I work greater than 40 hours a week at times, but less than 40 hours a week other times. Overall it balances out between my needs and the needs of my broader team. I have the support of a great manager, who has never pushed me to do any extra overtime work that I didn't already want to, but has encouraged me and the team to take time for ourselves when we have been working really hard.
I also think work life balance has a lot to do with how much you enjoy the work. I've worked remote for AWS for nearly 5 years now, and my particular role at AWS gives me a lot of freedom in deciding what I do day to day, from writing blogs and technical content, to creating sample code, engaging with folks on social media, recording videos, travelling and giving talks at conferences, providing feedback on internal product specs, testing preproduction releases before they go live, etc. So the variety keeps me interested.
Hope this insider perspective is helpful!
