"A naive business owner was aggressively sold into a plan that wasn't sustainable for their business"

I agree with you here. However, as someone who has been on the ad sales side of the equation, I can tell you that all sales are emotional and the more time you spend explaining to people how to properly price out the product the less likely they are to buy. Literally the more rational you make it for the person to buy your product the less likely they are to actually buy it, because the rational part of the brain isn't the part of the brain that buys stuff.

In addition, I'd argue that what makes advertising not-unethical is that the vast majority of businesses lose money on any given ad buy. If advertising was just a license to print money for all businesses instead of, say, the best 5% of businesses then it would be pretty hard to argue that advertising was making the world a better place. While Groupon may well have been at fault here, the small business owner does have some responsibility to understand how advertising works, what the best practices are, what asset they are trying to build, etc. If the business owner in question understood this then it wouldn't really matter that they didn't understand every last feature on the site. It sounds like her real problem was that the people coming into her store didn't like her stuff; the fact that she was losing money on every sale would have been a non-issue to begin with if this weren't true.