ToM, Prologue:

 

"It soon became obvious, even within the stedding, that the Pattern was

growing frail. The sky darkened. Our dead appeared, standing in rings

outside the borders of the stedding, looking in. Most troublingly, trees fell ill,

and no song would heal them."

I think it's because ghosts are still part of the pattern, like in TAR, but are just leaking into the physical world. And from what we know about stedding is that they are seperate from the pattern, in a way, which is why the OP doesn't work, can go there in TAR, etc. Don't think it's anything complicated.