I feel no sympathy or mercy for them. It's under five hours to Louisville from Chicago DRIVING, and they sold those seats to paying customers. Then United violated their contract for a *very* minor purported savings, which is going to cost them untold millions now. You may say that under a capitalist system, they can do what they did and it's expected. Perhaps it *is* expected at this point, but it certainly wasn't *rational* of them to do if they were acting in their own best interests. After all, failing to live up to your agreements given that you have the ability to do so is normally pretty damning, whether you're an individual or business - just look at Sears, where upon merely the FEAR that they won't be able to pay their suppliers we find that their suppliers are reducing shipments.
If it was SO important to have those employees in Louisville, BUS them, or rent a car, or ANYTHING but what they did. It's fine to *offer* to have people give up their seats - see Delta instead when they had storms and issues - but United showed horrible judgement and exemplified just how atrocious they can be (link).
I continue to hope that United becomes embroiled in a messy, public lawsuit where the public sees how they behave and they suffer dearly for it. We'll see how negative the amount "saved" by flying those employees can go.