I'm an ad guy. I make TV commercials in NYC. You're right, you're seeing a lot of this. There are two reasons for it:

1) Mixed race couples are the fastest growing demographic group. Not just black and white, but also white/Asian, white/Hispanic. It's about $$$, and moving the merchandise. It's direct targeted appeal to them from advertiser.

2) It's also a defensive move on the part of the advertister against the twitter outrage mobs. If you did white/white or black/black (or all white) they'd come after you for tokenism, or racism. (Such is life today. Some preening idiot is always going to find something to get outraged about) So you eliminate one source of potential outrage by mixing things up. And, you save production/talent money: A black/white couple (one black actor, one white actor) saves you money in talent costs. You cover the bases with a mixed race couple, and keep your talent costs down, while avoiding the outrage crowd.

No product manager wants the call from his/her boss about the Twitter mob. This is how you avoid it, and look virtuous doing it.

But ultimately, it's all about $$$$$.