Because realistically, most of those dual citizens started out as foreigners and were eventually naturalized as a consequence of pursuing an academic career in the US.
My guess would be that a large number of dual citizens obtained their citizenship in the course of their scientific work. It speaks to the training problem we have.
Why on earth would you group dual citizens in the same category as foreigners?
Even if 100% of the faculty are dual citizens (Americans + something else), it would still be 100% American.