Public DNS Providers could possibly abuse their position and see what users of it are doing on the internet. It‘s different though, because a public dns provider cannot see who the user is exactly, they could take a good guess at it, but it‘s not always certain. ISPs (atleast swiss providers) have logs of IPs to Customer. This would allow the ISP to see exactly what customer XYZ is doing on the internet. Even if it‘s not permitted by privacy laws, what stops the provider from accessing it? Public Providers could do the same, but they most definitely do not know the exact name, address and other details about the customer, as the ISP can.