The intention of "minimum wage" in the US is not merely subsistence level. FDR said, "by living wages, I mean more than a bare subsistence level-I mean the wages of decent living." [0]
The "iron law of wages" is instead an economic principle that wages tend to trend downwards until people are paid the minimum possible for subsistence. It's not meant to be a goal.
There are ten million stated reasons for the minimum wage! Pretty much the only one that economists can agree on is that it's helpful to prevent abuses of monopsony power in small towns.
Regardless of what FDR said, a living wage is guaranteed because people will not accept anything lower.
The problem with a "decent" living is that reasonable people can disagree about what that looks like. Roommates? Children? An unemployed spouse? Vacations and retirement?
It's not the government's job to guarantee all of that stuff and I would rather we focus on stopping wage and tip theft and protecting the rights of workers (banning noncompetes, decoupling health insurance, etc.) instead of increasing the minimum wage towards some poorly-defined goal.
There's also the other side of the minimum wage debate, which is that most of the specific numbers people list as "liveable" do actually result in some folks losing their jobs and becoming unemployable. There was even a recent BERKELEY study that showed this!
The "iron law of wages" is instead an economic principle that wages tend to trend downwards until people are paid the minimum possible for subsistence. It's not meant to be a goal.
0: http://docs.fdrlibrary.marist.edu/odnirast.html