The number of unique visits in a day is the number of total visits minus the number of repeat visits from the same users, so they need something like this to ge

The number of unique visits in a day is the number of total visits minus the number of repeat visits from the same users, so they need something like this to get an accurate count. You can't produce the number without information on repeat visitors. I think you are right that this technique could be changed and turned into a way to track individual users. But as implemented, it doesn't do that, and all knowledge is lost after one day. We shouldn't criticize people who are trying to limit the information they collect to the bare minimum by pointing out an altered version of their system might have undesirable properties.