Likely the hunter who did this didn't know about colic and just wasn't thinking that he was throwing feed into a horse field. I assume the fields are large (they look so in the photo) and if he wasn't a horse person and the ponies were in a distant, non-visible part of the field-- he might not have put two and two together and realized he was putting the corn in the pony pasture. If he's not a horse person, you just can't assume that things that seem obvious to us (I opened a gate, therefore I am in a used pasture even if I see no horses) seem obvious to him.
Regardless, this is sad and should not happen again. The club needs to make the rules abundantly clear to all members.
Sorry for your loss!