I don't know if it is currently posted not to camp on the meadow. It's difficult to post signage for the Sulfide because the trailhead is in National Forest. One doesn't enter the park until Shannon Ridge, and the only sign there is a small National Park boundary sign. Usually information about restrictions on camping are provided at the point where the camping permit is issued.
And now that you press the question, I'm not sure I'm recalling this right... I know I issued the guy a citation and lectured him for sleeping on the heather and providing such a poor example for his students, but the infraction that resulted in the citation might have been something else, like camping w/o a permit. It's been at least fifteen years since I was a Tool of the Man, and my memory is notoriously unreliable.