I'm not sure that everything you said is correct, but the beauty of at-will employment is that it allows most of these problems to be solved by people looking to their own best interests. I don't know many companies that would try anything like that because their entire workforce would likely just give notice and quit. That's not in anyone's best interest, so labor conditions (at least in reasonably in-demand fields) tend to be sane.
Of course, it's true that there are differences in the social contract you usually get working in the US compared to other places -- for instance, there is less expected vacation time here than in a lot of EU countries. But that's a tradeoff that people choose to make.
The entire workforce can give notice and quit in those conditions in Europe, with better unemployment insurance, free health insurance and much more chance of being able to extricate a financial settlement from their employer for unfair treatment.
I'm not saying nothing about at-will contracts filter down to benefit employees: US companies can often afford to pay higher salaries which on some occasions even correspond to higher hourly rates, and they can take more gambles on marginal job candidates. But it would be foolish to pretend it's a balanced tradeoff rather than a massive shift - fairly or otherwise - away from the interests of the median employee and towards the interests of the company.
Free in this context usually means free at the point of purchase (with contributions being based upon ability to contribute; which, in particular, means that for the unemployed it can be entirely free). Employers contributing is actually pretty common (see e.g. Germany or the UK). The key part is that healthcare is not contingent upon employment (or being able to pay for it independently).
Of course, it's true that there are differences in the social contract you usually get working in the US compared to other places -- for instance, there is less expected vacation time here than in a lot of EU countries. But that's a tradeoff that people choose to make.