These people probably work their ass off during their 40, 60, or maybe 80 hrs on the job. So they dont understand when they ear that startups' work schedule is more relax because they cannot relate to it. However, when they leave their desk, it's over, they're up to something else and they probably even force themselves not to think about work anymore.
Startups take a relaxing approach to work hours because the (right) person who works there lives and breathes startup 24/7.
It's easy to say when you're a founder (disclaimer: I am one). But it is something I have witnessed in (good) startup employees as well. They think about it all the time.
@falcolas is right, who the hell cares how many hours in the week you spent executing your tasks? Shouldn't the time "thinking" about work be valued as much as "executing" the work? Don't we all "think" better outside of execution time?