Why does Groupon use sushi for all its ads?

Maybe yes. Here in Brazil they have the same ad. And they include sometimes barbecue( I dont know if you have this kind of restaurants) and stetic clinics.

Always a 90% discount on sushi. Shouldn't they be sued for deceiving ads? These offers are never real.

My guess is that sushi sounds expensive, fancy, and gives colorful pictures.

Maybe the reason is very simple. Ad friendly (visually attractive), colorful, fun, practical, mostly not unhealthy and there's a sushi kind for almost everybody.