The rationale the Democratic party has thrown in opposition is SHAMEFUL. But to your point, insofar voting is free--it costs NO money. If it costs money to get an ID ($30 for my latest renewal), then voting is no longer free.
If one was to oppose the measure--which I don't, I've just maintained the rationale for implementing it has been whorishly trumped--opposing the sudden "poll tax" seems to be one of the few, if not the only irrefutable stance.