That's exactly my point, which is why it's dangerous. If the exploits were made public as soon as possible (full disclosure) then MS will have incentive to release bugfixes as soon as possible to the general public.
By default a naturally occurring exploit probably upon inspection looks less intentional then one put there on purpose.
. . . except for the outsider who mailed them about it and got paid a chunk of money as a result.
At least an internal employee is on your payroll, and has been screened with a background check. You don't get to screen which people get to discover vulnerabilities.
They would not know about the program which takes that exploit and then gives it to the NSA.
Nevertheless, I don't see how this is a bad thing.