The secret back door in Juniper Networks firewalls took advantage of a
weakness
introduced intentionally by the NSA in an encryption standard.
Thus, the argument that "If we let the NSA break it, others will take
it" is not mere theory. (People usually state this as, "The bad guys
will use it too", but I reject the presumption that the NSA is always
a good guy.)
Since the weakness has been publicly known for years, this fact does
not prove that the NSA introduced this particular back door. That
might have done by various attackers. It's also possible that the
back door was inserted by Juniper Networks, or by some of its staff,
perhaps in collaboration with the NSA.
Juniper Networks has fixed only part of the known problem. Because
the software is proprietary, users are compelled to wait and see if
Juniper Networks inserts the presumed one-line fix for the rest of the
problem.
If it were free software, these users would be free to insert this
change on their own.