Last year Michigan repealed their compulsorily helmet law, which went into effect in early April 2012.
Here's an overview:
Here's even more detail they offer (from the first page link)
I imagine right now many who have had their license for awhile are comfortable with a helmet and don't buy into the helmetless argument. It will be interested to check back in a few years as new riders start up and many of whom will not be using helmets because the law permits it.