The issue is the lying.

If you're going to eat meat it's no worse than others, the issue is that you should be able to trust that the food you bought contains what it says on the labelling.

Apart from the lying and profiteering, there's the issue of animal welfare and people's health. Horses are slaughtered in knackeries, not in abattoirs (which supposedly only slaughter animals for human consumption). Knackeries must comply with their state or territory animal welfare legislation but every state differs, so if there is horse meat in your mince you do not know under what conditions in was slaughtered. If you want to know a little more: https://www.farmtransparency.org/kb/food/knackeries

Healthier if it's legal and regulated, but these are just random horses. Some have medical issues, so have built up medicine in them that's not necessary healthy for humans.