Behave badly everywhere! I have spoken with couple dog owners and basically, all of them denied leaving dogs poo on trails.
They all seems to be genuine, honest members of public, so the conclusion is obvious. When nobody is watching, cyclists defecate in bags and leave them on trail! Disgusting. Shame on you cyclists.
p.s. Personally I think we need to blame mtb cyclists or cyclocross, as road cyclists can’t ride on trail. (or at least is disgrace for roadie) 😉