I don't think they really get "attacked" so much as everyone tried to explain why horse slaughter is a necessity, and because it's so many people it can come off as rather hostile. You say that we aren't slaughtering dogs and cats.. Well, yes, we are. Shelters kill hundreds of dogs and cats across the country every day. If a person can't keep their dog, and nobody will buy it, their only option is to bring it to a shelter where it could likely get put down. Similarly, if someone can't keep their horse and nobody will buy it, they've no choice but to send it to slaughter auctions.
It is awful. But if we didn't slaughter animals - horses, cats, dogs - there would be SO many animals in awful homes being neglected because they can't be taken care of and fed. Unnecessary cruelty should certainly be addressed (and here I'm not referring to the way they kill the horses - that can't be helped, but I'm referring to how they are treated before slaughter) but overall I don't think members here try to attack them.
Sometimes people do get sick of someone coming along with a "OMG horse slaughter is so mean it should be banned, they beat the horses, etc etc" kind of post, because that person has obviously not done any research about it. In those situations, I do agree we may come off as a bit snappy... But I don't think anybody is intentionally trying to attack any body.