I don't see anything wrong in working agreed work hours, and completing a reasonable expected work amout and that's it. I think that's just recognising you have family and other responsibilities.
A lot of companies think your not motivated if your not doing 80+ hour weeks, for little added gain. Your not a founder, or have little or no equity and so have a limited upside. I don't why companies expect founder level commitment for non founder level upside.
In this case your biggest duty is probably to your loved ones and family. And not sacrificing that over some misplaced sense of duty. I'm sure a lot companies would not do the same if the positions where reversed. They'd probably fire you pretty fast, if you had issues effecting work. Keep that in mind.
It's the classic rhetorical: why not just work consistently, and not have to worry about cramming? Why do people behave that way (in either school or work)? In my personal experience, I've found the only time I cram, is when I'm stuck doing something I really don't care about (happened constantly in school).
One or two demotivated people? Well, okay, they're probably to blame.
But when you have a company full of them it's a toxic company and poor management that caused it.
Some of us are happy with working at a steady pace without many external symptoms of exctitation. I even believe this is better in the long run, people don't burn out and are better-rested.
Perhaps by doing their job to a satisfactory standard and abiding by the conditions of their employment?
If that's not good enough, the contract should make it clear just what the job really entails.
I'm less surprised by out of touch management once in a while and more surprised by individuals so confident in their demotivated lifestyle and their ability to maintain it long term with no consequences.