A 'tiny dog poo' could turn a kid blind. Horse poo won't.
Also, the most dog poo is deposited in inhabited places, often where people run and walk, and kids play - like parks and pavements. Horse poo is generally dumped in rural areas, where few people are walking, and it washes away fairly quickly when it rains.
Horse poo is pretty harmless stuff - as harmless as mud. In fact, given the stuff sprayed on some crops, possibly less harmless than mud....
Still stinks and attracts flies on hot days. Id be more annoyed if I stepped on a horse shoot than a dog one lol, although Id imagine I would see the horse shoot!
Oh and police horses definitely drop it in inhabited places

On the blindness bit, is it not worm eggs that cause the blindness, I remember this made the newspapers but I dont think it is all that common. Do horses not get worms?