It is legal as long as it isn't used to manipulate the pps.
Let me better clarify:
It is hard to distinguish NSS from borrowing shares because it is basically the same thing; that is why it is hard to prove. But, in its full form, yes, NSS would be considered illegal because the intent would be to drive the pps down. Does that kind of make sense because I thinked I just confused myself?!?! LOL