It would not be that much of a problem if ads promoted healthy and tasty food but they will probably promote an ice-cream made from a powder and chemicals emulating taste of berries rather than from milk and fresh-picked berries.

It still would be. Loss of agency. Ads are text and images you see. Native advertising in a chatbot conversation is a third party bidding their way into your conversation. Machine showing you an ad versus injecting intention into your context are very different things.