The flag was "fluttering" because the astronaut who planted it
disturbed it, and in a vacumn there was no air resistance to stop
to fabric from fluttering (the only thing being the friction at
which it is connected to the frame). Hence any fabric disturbed in
a vacumn will "flutter" for a lot longer then in an
atmosphere.
And frankly if NASA spent millions to hoax the moon landings, you
would think they could get such an obvious detail correct. There
are many other theories on other things (like photographs taken,
lighting) and all that other stuff that is at least more
compelling, but then again there are very good answers for
them.
