Because other cultures do not necessarily share widespread American feelings about the use of horses as human food, the ethics of sending horses to slaughter can be legitimately debated. But I think both pro- and anti-slaughter advocates can agree that no living horse should be transported anywhere for any purpose without being provided adequate food, water and care while in transit. Even chickens deserve better than that.
It should be noted that the ban is not necessarily permanent; should future inspections indicate that the transport issues have been addressed, the EC's ban may be lifted. But for now, a well-deserved slap has been given to those who would rather scrape out the last penny of profit than provide basic care to the animals in their hands, and it is aimed at the only place such people will heed: their wallets.