Ironically, this is a very simple reason: a creature affected by polymorphous must have a CR or level greater than or equal to the challenge value of the creature being transformed; and the players do not have a challenge ranking. And since they do not have the same statistics as what they've processed, it's not appropriate to just stick the CR of the form on them.
So, a druid who took the form of a black bear, without challenge rating, would not be eligible to be the type of animal that a creature targeted by polymorphous could turn into.
The main restriction on polymorphous is "a beast with a CR equal or inferior to that of the target", without the same stipulation found in Wild form that this must be a beast the caster has seen before. As the druid's NPC has a CR (and it was probably calculated with alternative forms that he might take into account), the real question is whether
- The Druid NPC counts as a beast in this form, and
- the polymorphous the spell allows the target to be polymorphic in a "specific" version of a beast, and not just in the platonic ideal of a beast, and
- The druid NPC of changed form represents a specific version of a beast and not their platonic ideal.
For the first question, I think the answer is yes, they do indeed count as a beast, given the answers to questions that are based on a similar logic.
For the second question, I think the answer is also yes, it turns the target into a specific animal. There is no text to contradict this, and even the specific non-demoniconic beasts found in the Adventure modules have a challenge level associated with them, and are generally valid targets for Wild Shape and polymorphous.
There remains the last question: does a NPC Shauide Druid represent a specific version of the beast they took the form or a generic version of the beast they took shape? And I think the answer is also yes: they represent a specific version of the beast. They are a beast with the same statistics as the form they took, except with (probably) higher mental statistics.
So, as far as I'm concerned, it's legal to polymorphous a creature transformed into a druid of altered form or wild form (NPC), and doing so will result in the polymorphic creature inheriting the druid's mental statistics.
At my table, because of that decision, I think I could then extend it to allow that to be valid if the druid is a player character; but as I said, I do not believe that the rules as such are valid.