Whilst it's likely that the horsemeat isn't unsafe, the fact that entirely the wrong type of meat is entering the food chain means that the way in which those animals were treated is also unclear. Horses can be prescribed a number of different steroids that are not suitable for human consumption.

More broadly, the fact there is nothing wrong with heating well-reared horsemeat, isn't the same thing as people finding it acceptable. Dogs and Guinea Pigs are eaten in some countries too; however, they, along with horse, are not deemed 'acceptable' meats in British society.

Where I agree with the comments, is that people (in the UK) may post-rationalise their position by saying, "There's nothing wrong with eating horsemeat." However, this would be a classic example of behaviour driving attitudes (rather than the attitude that it's OK to eat horsemeat having been present previously (something people do all the time).