I think you're right. This is a function of Marketing departments wanting to create large audiences. Large audiences mean more chance to attribute a sale, and

I think you're right. This is a function of Marketing departments wanting to create large audiences. Large audiences mean more chance to attribute a sale, and also more impressive sounding campaigns. The technical and time cost of implementing something that removes people who have made purchases from the audience (probably 0.001% of an audience) is much higher than simply adding someone who visited your website (lets say 5% of the audience) to the audience. If your product is reasonably ok, your largest group of customers is often your previous customers, so attributing the ads those previous customers see to their next purchase is logical for marketers.