All good points. I'm seeing more questions about this at alot of trails, I think better signage is in order, we'll see what we can come up with.
As far as why people walk/run on some trails, I'm with you, corkr, scratching my head. But the truth is that non-bikers often haven't any idea what they are looking at in terms of direction, speed, or style of trail. That mound of dirt that is so obviously a jump to you and me is just a mound of dirt to them.