If There is a Social Obligation to Pay a "Living Wage," Why Does It Fall Only on Employers?

Jason Brennan at Bleeding Heart Libertarians raises some tough issues surrounding attempts to shame Wal-Mart (or other low-wage employers) for not paying employees enough to constitute what some consider a "living wage":

Now, suppose Bob works at fast-food chain McBurger in a competitive market economy where he gets paid his marginal product, $1/hr. Suppose that he therefore qualifies for government assistance, receiving an earned income tax benefit or basic minimum income, food stamps, and the like. Many on the Left would say that the government thereby "subsidizes" McBurger, because McBurger pays Bob less than it takes to keep him living well, and the government pays the difference. But this presupposes that if you hire someone for, say, 40 hours a week, you owe him enough money for him to lead a decent life….

Isn't it more plausible to think that if there's some enforceable positive duty to provide Bob with enough stuff to lead a life, that all of us, together share this burdensome duty, rather than just Bob's employer?….

Imagine you argued for the following principle: "If you hire someone full-time, you have to pay that person enough to lead a decent life (defined as follows…), even if that person is so unproductive that you lose money by hiring him." That kind of moral codes gives potential employers of the unproductive two options: 1) hire unproductive people at a financial loss, or 2) refuse to hire unproductive people. It forbids the middle ground–help out unproductive people (perhaps even, in the process, helping to make them more productive) by paying them what their labor is actually worth.

*Thus, a tip to people making the Walmart subsidy arguments: your arguments would be much better if you could establish, empirically, that Walmart employees are getting paid much less than their marginal product as a result of a market failure. 

Reason on "living wage" arguments.