I hate showing ID just as much as the next guy. (Well maybe not as much as John Gilmore, one EFF's founders,
who sued the federal government
when he was denied boarding because he refused to show any ID.)
But as everyone knows sometimes it is background facts - i.e., not the facts of the case, but the facts in society - that drive decisions. And here, the background is the changing state of ID requirements in the U.S. Over the last 30 years or so, the US has created what is more or less a de-facto National ID card system, of mixed private and public parts, as Michael Froomkin pointed out in a 2004 paper. As the ID-requirement has spread into so many areas, like the entry of public and private buildings, it makes Indiana's law look reasonable, or at least something hard to decide on assumed facts.
So in part, politics; but in part, a battle over ID already lost long ago.