When someone visits VoteRef, they are greeted by a map of the U.S. and a search bar. All a user has to do is then search for someone by their name or lookup an address. 404 Media found many friends, colleagues, and other people in the data. In many cases this includes the address they provided when registering to vote, their age, and party affiliation. Ordinarily if someone wanted to access this voter data, they may need to go to each state body individually and purchase or request it. VoteRef bypasses that. And when someone provides their address when registering to vote, many are probably not aware that their address will end up publicly available on this website for anyone to look up.
And as with so many things, you can stop voting (I know a bunch of us have), but that doesn't delete the info from the past.