The US public has a legitimate interest in knowing about NSA's hacking abilities, because vulnerabilities used by the NSA abroad can just as easily be used domestically by foreign or criminal attackers.
Against a well-prepared adversary, a vulnerability can only be exploited once -- a zero day. After disclosure, or evidence of an exploit in circulation, such vulnerabilities become useless towards the agency's mandate, whether or not you agree with that mandate. Realpolitik? Definitely.
