"Since when did management become a 40 hour a week job ,or less, for some?"
Since I set the expectations when I started working for the company. In 33 years, I have generally put in 40 hours a week as an exempt employee (aka manager, whatever). There have been exceptions, but they have been few and far between. My managers have always been happy with my work, my output, and work ethic. I have always gotten great reviews.
The key is set the expectation from the start. If you go in and want to "wow" your manager and start by working 60 and 70 hours, then that becomes the expectation, and will stay the expectation.
Work your 40 hours a week, do an exceptional job, and say "no" to working the extra until there is the rare and real emergency. That's when you give the extra. That's when you are seen as generous, loyal, and a hero for the company, and your contribution is appreciated.
If you are working 70 hours a week, do you really have extra to give for that emergency? Probably not. Then YOU are the one seen as a slacker, while I work 50 that week and my director or AVP loves me.
I don't like RTO, but I'll play the game, and I'll continue to work my 40 hours.