This is correct and intended behavior I believe.
By default, packages archives shipped on the ISO images are signed with
the cdimage key, and are trusted.
This means one can perform [dist-]upgrades from installation ISOs (note
these are no longer produced by default).
And enabled one to install packages from the ISOs (e.g. install updated
openssl on a 14.04.0 host from the 14.04.1 server iso; another example
install optional drivers off the desktop iso).
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ubuntu-keyring imports the Ubuntu CD Signing Key as a key trusted by
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