I seriously think that we should be able to charge people who get on a horse when they are too heavy for the wellbeing of the animal with animal cruelty/endangerment/etc. It's so completely horrific and as someone who has ridden horses for over a decade and a half, it makes my blood boil to see 300 lb. idiots getting on the backs of a 1400 pound horse.  There are so many negative repercussions for the horse that can happen after even just 15 minutes of strain like that.