The problem with this reasoning is that assuming the content of the ad is not discriminatory, delivering the ad to some subset of your audience is highly unlikely to be illegal. For example, if you're a publisher that owns two magazines, one of which is popular among blacks and another is popular among whites, I don't think it's still illegal to publish ads on one but not the other, again, provided that the ad itself does not have illegal contents. The onus must be on the advertiser because the full extent of the targeting for the given campaign is not knowable for the publisher. Maybe the advertiser wants to reach men on one publisher and women on another publisher, this isn't something any given publisher can fully determine. Given how advertising works and has worked in the past, it's unlikely that any law would put the responsibility on a publisher for an entire campaign to be free of illegal discrimination on the basis of an audience it reaches.