Good question! I saw someone else around here recently who fed their horse fruit as part of their diet a few times a week. It didn't make me question it, it made me wonder why I never did it. I had a mare once that would mug you for a banana. But other than the apple here and there, I never did.
I know some people feed bananas and my dad used to have a horse that would sneak into the apricot orchard and eat the apricots. Apparently he used to make a really funny noise when he spat out the pips!
We tend to feed our 2 horses a bit of produce in general. I agree- I think they would readily eat most of it in the wild, so why not, in moderation? They get apples from neighbors and my husband's co-workers. I tend to treat them like living compost piles and feed them fruit and veggie scraps that aren't bad- apple middles, cucumber skins, cabbage bits, watermelon rinds, etc. if they don't eat it, the dogs will once they drop it, thinking it must be horsey gold! I fed them some watermelon rind that had been frozen a week or so ago (I had frozen it for them). My mare could care less that it was squishy, she was still going nuts for it.
Fruit is good for them in moderation. I've fed everything I have ever brought to the barn, apples, watermelon, grapes, banana, carrot, snap peas, etc. I have always been told to keep some things very low in moderation though, like granny smith apples, because they make the horses gasey. If it is true or not, I don't know, but they arent mine so I do as requested lol.