It is, but they also have a competing mission to compromise the security of non-American businesses, communications, and infrastructure. Since for the most part, the same software is used by both, these missions often contradict each other. e.g. documents from Snowden revealed that the NSA had broken encryption recommended by NIST. It's difficult to know if NSA recommends something because it makes their defensive mandate easier, or their offensive mandate easier.
For this reason, experts like Schneier [1] have long advocated that the NSA be broken up so that these two missions do not fall under the same agency.
NSA's defensive mission is about securing National Security Systems. These have a fairly specific definition, and are not running in the average business.
