It may hurt productivity, but the employer gets the benefit of keeping the employee on a leash by not giving them time to look for other opportunities , plus in some businesses you can just keep grinding employees to the bone and sift until you have the most workaholic ones, such as in finance.

I won't claim to know what will happen to Amazon or other companies if they keep trying to squeeze every cent out of every employee all the time, but in my professional circles of people who work front office at investment firms or consulting firms (in the northeast), there is a long history of making sure the newbies are in the office 80+ hours a week. These are also the same firms that continue to make the most money year after year, so I would assume there is some possibility to extract extra value from the subset of the population that is workaholic.