I am well experienced with horses AND donkeys for that matter and I dont see how this could be a cruelty to animals in any form. Yes, horses play in all sorts of sports that include balls. Like polo for instance. If they're growling over this fun sport, they might as well band polo. For the rider must carry a long pole and hit a small ball around on the ground while riding a horse. This is just a fun activity that they want to ruin for kids.
If they want to growl at something. Why not growl at horseslaughter or globalwarming .. something other than this ridiculous attempt!