1. They were probably relying on those exploits quite a bit and had frequent success with them, in which case totally burning them would have directly harmed operational capacity.
2. They appear to be huge fans of piggybacking off of other intelligence agencies' footholds and data ("fourth-party collection").
http://www.theverge.com/2015/1/17/7629721/nsa-is-pwning-ever...
>Fourth party collection appears so successful that agents of the NSA and GCHQ have cracked jokes about it in top secret slide decks. In an NSA presentation titled "fourth party opportunities," the first slide references Daniel Day-Lewis' infamous "I drink your milkshake"
Another intelligence agency acquiring these exploits would give them additional "fourth-party opportunities". As long as the US government's systems were protected against the exploits (were they? no idea), that could mean keeping them secret still meant more pros than cons for their goals. It could give them more intel, and maybe give additional insight into the other nation's goals and interest by who they're targeting.
Pure speculation on my part, though. The real reason could just be regular old incompetence, and/or internal cover-up of the tool leakage.
Furthermore, they can't really keep using them after exposure for the same reason the Russians didn't start using them: everyone who knows about them can watch for them being exploited and so gain useful intelligence on their rivals.
How do we know they don't? Or at least, some of their exploits.
Also, that's not necessarily the only conclusion. This leak could have have been more harmful than no leak, yet thought to be less harmful than publicly burning every exploit.
>Furthermore, they can't really keep using them after exposure for the same reason the Russians didn't start using them: everyone who knows about them can watch for them being exploited and so gain useful intelligence on their rivals.
True, but they could possibly change them in a way to make them less detectable.
The Russians could have done the same, but maybe their reason for believing it'd be helpful for fourth-party collection is that they already had access to some of Russian intelligence's communications and compromised hosts, devices, and networks.
Anything could be possible here, though. We're all speculating pretty blindly.
The principal rival in the short term for any department whether of government or inside a company is other departments not the corresponding department in another company. In fact it is plausible to think that at least some of the people within agencies like the NSA have more in common and a substantial degree of fellow feeling with their nominal rivals in, for instance, Russia than they have with other US agencies.
I'm not saying that I personally would reach the same conclusion, just that the situation is more complex than Reuters makes it out to be.