The National Defense Authorization Act probably did not authorize US police or military organizations or individuals to detain US citizens without cause or due process. The bill was written in such a way that existing statute in this regard would not be affected. But just to make sure, President Obama added a signing statement stating that the new bill would not be used that way.
The relevant paragraph states:
The fact that I support this bill as a whole does not mean I agree with everything in it. In particular, I have signed this bill despite having serious reservations with certain provisions that regulate the detention, interrogation, and prosecution of suspected terrorists. Over the last several years, my Administration has developed an effective, sustainable framework for the detention, interrogation and trial of suspected terrorists that allows us to maximize both our ability to collect intelligence and to incapacitate dangerous individuals in rapidly developing situations, and the results we have achieved are undeniable. Our success against al-Qa’ida and its affiliates and adherents has derived in significant measure from providing our counterterrorism professionals with the clarity and flexibility they need to adapt to changing circumstances and to utilize whichever authorities best protect the American people, and our accomplishments have respected the values that make our country an example for the world.