Leaving your hitch and ball in place defeats the purpose of having a 5 mph safety bumper, and constitutes neglect on the owners part. Yes, I pay attention when walking. Ever wonder why stairs are done in bright yellow paint? There's your sign.
Sure can tell who is lazy... a judge would pretty much ask "is it foreseeable that while not in use, someone walking through a public parking lot could get injured by leaving it in place?" Enough here have answered "yes, it is foreseeable". Standard test of reasonableness. Remove it when not in use.