I think the problem is that "data science" means many different things to different people. To you it connotes applying statistics to marketing, but for others it covers large swaths of probability, statistics, machine learning, even things like geometry, etc.
But this can be an opportunity too. If I wade into the politics of hiring just a little and also interpret your question as "why would data science professors be principled additions to math departments?" ... well, if a department can secure a line of funding for "data science", they may not choose to hire a marketing or advertising person, they might choose to fill it with a probabilist or so on.