So instead of opposing the entire idea of voter IDs, why do we not pass a Federal law that makes getting an ID free of charge?

Seems like that's the root cause or the main contention.

There's a big can of worms here. The thumbnail sketch is "Americans have some (as viewed from outside the US) odd and severe hangups about being tracked by the government that is, ostensibly, theirs."

Reasons range from the practical / legal ones listed by the ACLU (https://www.aclu.org/other/5-problems-national-id-cards) to a small-but-vocal subset of voters who actually believe (because so much of the US is descended from Christian zealots fleeing persecution in their home countries for heterodoxy) that a card issued by your government that is required to participate in society is a literal "mark of the beast" as per the biblical Book of Revelations and therefore something to be resisted as part of a struggle against anti-Christendom.

No, I meant, passing a law that says "$0 for all state IDs". Not talking about National ID.