I've never had a child draw on anyone, and I'm usually pretty in my child's face about discipline. So, it's unlikely, but not impossible, that he might act out even when I'm right there.
But, just because something is bothering you, it does not necessarily mean that it is something I should be disciplining or something the child should "cut out." Maybe it is (about to use a marker on someone), and maybe it isn't (looking.) When the parent is right there, it is inappropriate to be intervening unless you or he is in danger and the parent is incapable of reacting to the need.
It's a parent job to discipline their child, and when strangers do it for them they both let the parent off the hook and potentially intervene inappropriately with the child.