NSA doesn't actively monitor US citizens without a warrant. Our info may be passively collected though in their digital dragnet. Sometimes they send tips to law enforcement to bust illegal druggies and human traffickers which are missions of theirs.
If politicians redirected the full capability of the fed gov from going against bad guys over to us, the oppression would be unbearable. There are so many laws from local to fed, it's literally impossible to follow all of them. Thus pols have turned all of us into criminals waiting to be prosecuted for something.