Barbara, 
I agree with you...  However,  the common trails that hikers, mountainbikers 
and we riders use are so precious - and falling in numbers.  I have no 
problem with kicking some poop off a trail in order to continue to please 
others around me and have less animosity towards horses.
There are many a landowner here in the Northeast that just say no to horses.. 
People ride with soft trail conditions (spring & after rainfall) and rut up 
hiking/biking trails that these people who own the land use for their own 
purposes and not necessarily horseback riding....
I believe that we should be respectful of others and if they don't want the 
poop in the trail..  Kick it out - - or stop using the trail..  
Times have changed - and we must accommodate land owners or suffer...
Debbie