It's worth noting that horses were probably domesticated as meat animals before they were ridden, and remained an important dietary component for the steppe peoples. Also, I don't think it is correct to talk about a general Western European taboo on horsemeat: while in many countries such a taboo exists, horsemeat is largely considered unproblematic in France and Italy (and I think Spain as well) although of course it is still way rarer than beef and other meats.Sentimentality - Horses are in that awkward halfway point between farm animals and companion animals. A number of OTL countries find the idea of eating them as distasteful as eating pets.
Religious - I believe the original reason horsemeat was taboo in Western Europe is something to do with pagan ceremonies involving eating horse that Christendom wanted stop. I'm pretty sure it's not kosher, either. I don't know how Caraíbas's Universal Religion might feel about it, but it's a possibility.
Health - During the UK horsemeat scandal one of the points was that horses are filled with drugs that aren't tested for human consumption, because it's assumed no-one is going to. The Societist bloc may not care so much about standardised dietary practices as standardised animal and human health legislation.
All of these possibilities would be a result of Societist standardisation, since obviously the Carolinians themselves don't feel that way, but it wouldn't be as simple as "Societists ban horsemeat because That Would Be Different".
(and yes, it's not kosher - I think it's halal, however).