I don't know if it's this way everywhere (I'm in GA), but I remember when I renewed my license, on the electronic form there was an already checked box that said something like "if you do not wish to register to vote, uncheck this box" or something to that effect. And it was in very small print. So, when getting a license, the default was to register to vote, and you have to specifically uncheck the box to not register. I remember this clearly because I had to uncheck the box every time I renewed my license (didn't want to register to vote until 2016). Maybe that's how it happens.