I think most people are angry that burger meat is being represented as beef, yet is actually horse meat. That, in my opinion, is fraud. Misrepresentation is fraud.
There is also the problem of veterinary medicine that may be present in horse meat in higher concentrations than in beef, although all factory-farmed meat (which you get in restaurants and fast food chains) contains antibiotics and other veterinary medications, as well as hormones to promote fast growth. It also seems likely that healthy horses are not being sold for this purpose since they perform many uses for humans in living form. This means that old, sick horses are most probably the ones that end up in burgers! "Sick" could mean any disease that affects equines, including cancer. I don't eat beef in any form, burgers or otherwise, but I do have family members, including grandchildren, who do, so it concerns me.
Then, there are those of us who feel that horses should not be eaten as food, and that may be a cultural issue, but it promotes strong feelings nevertheless. Those of us who live in cultures that do not consider horses a "meat" for humans to consume have a right to our opinions about this matter and to expect vendors not to fraudulently "slip" ground horse into what is supposedly another product.
This issue was exposed in the U.K., but I've read accounts of horse meat found in a Burger King in California, USA. This fast food chain has lost the trust of consumers, which causes boycotts. Their deceptive attempt to save money on the cost of burger meat may end up costing this chain their business if they lose enough customers. I can only think that would be justice. Consumers deserve the truth about their food supply. That's becoming more difficult by the day to receive, especially in the USA, and the action of boycotting is our best means to address it since our government agencies (FDA and USDA) which are supposed to protect us, but actually look after the interests of Big Agribusiness and the pharmaceutical industry instead, do not!
Okay...I'll climb down off my soapbox now, but you did ask! Ha-ha.