It's because it violates privacy. For example if someone has a huge secret hobby of collecting dildos and suddenly on some unrelated site they see an ad for a h

It's because it violates privacy. For example if someone has a huge secret hobby of collecting dildos and suddenly on some unrelated site they see an ad for a huge dildo.. that will cause a bit of an alarm. That's the extreme case, more than likely people are a bit unnerved when an unrelated site shows them something they looked up on amazon an hour ago. Even so I suspect a lot of these complainers just have hidden internet hobbies they just want completely private.