You ask, "is being stuck in a mortgage with a sick spouse license for an employer to demand 70 hours a week?". Well, at 70 hours a week, our protagonist is effectively being paid $19.78 per hour. That's almost $2 higher than the average income of employees in Grand Rapids, MI.
What you're effectively asking is, should it be lawful for employers to pay people less based on their situation?
On the one hand, there are plenty of circumstances where it clearly isn't lawful to do that. For instance, you can't pay people less because they're of Indian origin, or because they practice Judaism, or because they're female.
On the other hand, the circumstances of individual employees play into compensation decisions all the time in every job. You are effectively being paid less because of circumstances mostly out of your control any time you take a job outside of San Francisco or New York; the premium earned by technology workers in San Francisco exceeds cost-of-living adjustments significantly.
So where do you draw the line? Is the line "you can't be paid less because of illnesses in your immediate family?" I'd agree with that rule, but how often does it really come into play?
For what it's worth, speaking on behalf of an employer: we don't like 70 hour weeks. We work hard to keep them from happening and, for the most part, people get out the door here in time for dinner (it's hard to say, because some people stroll in the door just in time for lunch). Making people work overtime here, paid or not, is a bad idea because it makes it hard to retain talent. I'd like to think that if there was an economic rationale for overtaxing our team, we still wouldn't do that because it's immoral... but my morals haven't been tested on this issue, because it would be irrational of us to coerce people into working overtime.