It's a free wifi service provided by the airline while on the flight. One ad, when most other places are charging $10-20 per flight? I'll watch the single ad. I'd rather that then someone, say, injecting adverts into sites and services. which is very possible.
Agree, which is why this is mildly infuriating and not worse. Of course there were about 10 minutes of unskippable ads on their media screens before take-off which was a pleasure to be subjected to. That should get everyone free WiFi without needing an additional ad..