I fish the local canal and am constantly getting caught up on bikes, shopping trollies and other junk.
I have never had the chance to see or smell someones doo doo so i suppose i must be lucky or someones trying to leave a message that this peg is rubbish !!!
Seriously though it is annoying but if you dont clear their rubbish up then the person after you could think it was you that left it.
I carry a roll of cheapo bin bags in my kit precisely for that reason and it dont take no room up.