This is funny. (no offense to the op, I just mean that I share your grief).
We went to WPPI in Vegas with my core team (5 of us at the time) and asked someone to take our picture. Everything was set up beforehand. You would think that at a convention for wedding photographers, that we would be able to find a photographer competent enough to take the shot with proper leading lines, composition, etc.... HA no chance. We eventually found someone who was nice enough/interested enough to have a mini lesson on composition from our lead guy and we got our shot.
The point is, what was obvious to all of us as photographers was not clear to any other photographers at this convention FULL of photogs. I'd lower my expectations to avoid the frustration but be open to guiding someone through to get the shot you want. I also wouldn't expect that because someone is carrying a dSLR that they know how to use it.
Again, I feel your pain.