Just because they might not have the ability now, doesn't mean they're not planning to have it soon. http://www.dslreports.com/forum/r27012462-Wired-article-explains-NSA-decrypting-plans-new-facility
If all you need is a private key to import to a bitcoin wallet, why would the NSA bother "mining" bitcoins when they could just open up all the wallets in existence and send the contents to themselves or a black hole?
They wouldn't do that because it would tip their hand. One of the prime directives of a code-breaking organization is not to reveal your capabilities until it's really necessary and then ideally use the information in such a way that it does not reveal that you can do it.