Only they (companies) do expect it, and even get it. Because the employees are dependent on them, and not vice versa.
Those that say: just quit and get another job (mostly white, upper-middle class), have never been in the situation of having to keep a job to feed a family and/or mortage while not being someone coveted by recruiters.
If people spent even just a few moments thinking about their own future and prospects they might decide against purchasing a home, having kids, getting a new car and so on.
"Why don't they just leave?" is not a valid argument.
A consequence of reducing your long term career risk may be that you will have lower income (at least in the short term). You may believe that individuals have the right to maximize their short term income; but does this desire impose a responsibility on their employer not to fire the profit maximizing individual?
Sure, they can sacrifice all their lives -- including the American dream of their own family, home and car -- just to be better positioned with employers. No biggie.
Besides, I'm not saying sacrifice anything... if they wish to have the family, home and car they should seriously think if they can afford it.