If you live in the countryside next to a country road or near a bridal path then you're bound to see horse poo on the road now and again. It would be very unsafe if a horserider stopped in the middle of the road everytime their horse did a poo to clean it up. What would you rather, come round the corner and get some horse poo on you're tyres, or go round the corner and get half a ton of horse through you're windscreen?
It does get cleaned up in most cases, people will come along and scoop it up and then sell it to gardeners for quite a nice little profit. There is use for it, where is the use in dog poo?