Web owner should be aware of what shows on their pages.
For years I have barred certain 3rd party sites from delivering content to my computer. I also choose in security setting that only content from the site I am visiting shall be displayed in my browser.
If a web service places adverts hosted on their own site I will see them and won't bother to block them. Usually these adverts will be reasonably targetted anyway.
I take offense at any site that attempts to peddle shit from other sites without consideration to what may be offered. Any responsible site owner should want to know that they are not advertising immoral or illegal items on their pages.
Web site owners, take heed. It you place antything on your site, you are endorsing it. If it is illegal, you can be held responsible.