The issue is identification. Lots of people aren't good at differentiating poison oak/ivy/sumac from other non poisonous plants, and it could easily be abused by claiming a bush is harmful when it's really not. You also run into issues like people who aren't affected by urushiol, do they still get relief? I have some major pollen allergies that can cause rashes and hives, do I get relief from flowers? Do you get relief from every plant on every course in Arizona, ruining the only thing that keeps those desert courses from being completely wide open?
It does suck that I've lost strokes taking relief from poison oak and ivy, but I do understand why they aren't automatically included. I take much more issue with TDs who don't take the time to mark and give relief from the stuff at their events.
Not impossible to overcome. Everyone has a smartphone with google in their bag nowadays. If I were to play tournament at most area courses in summer, I'd be out 2-10 strokes to move my lie per tournament to avoid serious ivy, or, play from it and go to the hospital after. Not much a choice. I won't play.