I don't know the "rule" but I would think it's okay to set boundaries for this. Things like no drop ins during meal or nap times and/or must take child with them. If they want to "surprise" the provider and do a check in (which I agree with during the first 1-2 months) than I suggest to do so by picking up early.
I think if a parent is worried about another parent dropping in, than group care is not for them.
I personally don't have any times that a parent can't come by but if it will be during nap I ask to know ahead if time so I can make alternate napping plans for that child that day (I'll have them sleep in a different area usually so I can easily get them when needed without disturbing the others)