DY: The site puts the third-party ad generator (something like Google Advertising) on its site allowing that third-party to generate random, supposedly relevant, ads on its site. Without the site putting the third-party ad agent on its site, no third-party ads would appear on the site. Thus the site does bear responsibility for what appears at it has the option to not use a third-party ad service and use only internal, and approved, ads.
Sam: Short-sleeves is forbidden to look at and thus cannot be used in ads. Irregardless if that supposedly might attract non-frum people to look into frumkeit. You can’t do something that causes frum people to sin because it might make non-frum people frum.