Personally, I am not offended by eating horsemeat, nor do I think consuming normal horsemeat is harmful, but I can see why other people might be really upset by that. I think the overarching complaint is that people are getting something in their food that they did not expect and did not want, regardless of the potential harm to their health. The fact that it's horsemeat just makes it even more repulsive to some, even if they know absolutely nothing about horsemeat. (Just imagine: if it were bison/buffalo meat, would there be as much of an outcry?)
Nevertheless, I think a punishment is appropriate because there needs to be a precedent set regarding the honesty of food labels. Namely, food labels should be accurate and people should not be exposed to "surprise" ingredients, even if a contamination is accidental and not actually harmful to anyone.
It might be easier to understand this reasoning if we were to imagine that something bad really did happen. What would happen if someone who consumed the horsemeat were allergic to it and suffered seriously harm? Or if the horsemeat turned out to contain dangerous chemicals that killed someone? There would be no end to the litigation, not to mention the dollars spent on subsequent lobbying for food label reform and greater oversight in the food distribution and production processes. It would be much easier to prevent such an occurrence by sending a strong message right now: don't foul things up or you will be punished like the contaminated beef sellers.