Both, but the studies were literally prevented from happening or those that were done anyway then the results shared with Americans - the USA threatened to boycott the WHO iirc if it did not remove language to the effect that sugar could be dangerous, in excess.

HFCS lowers your metabolism, so makes every additional calorie count for a greater effect.

Stores sell what they want to sell, in part based on what people will purchase (e.g. fast food companies like McDonald's tried offering healthier options such as salads - people wouldn't buy them), and things with higher shelf life. They aim for profits, not service for its own sake.