To apply "Junk Mail Filtering" requires scanning the contents. Even doing the basic things like checking DKIM and SPF to just do basic validation requires reading and extracting data from the message and storing it for metrics/heuristics is an important thing all modern email systems do now. And this ignores even the fundamentals of delivering the message.
This is not black and white situation. Email systems need to read email messages to make the system work but they also need to read the email to do ads.