Usually you have to have a trial to prove someone committed a crime. The 2012 NDAA allows for indefinite detention as well as deportation of anyone the government determines to be a terrorist or a "substantial supporter" of terrorism. These people don't get a lawyer or a fair and public trial, both of which are legal rights of all American citizens.

Also, the NDAA is passed every year to determine the Defense department's budget. But this particular one was controversial because it included the provision schutzstaffel described.

Now, the criteria for determining whether someone is a terrorist or substantially supports terrorism is not as broad as described in the original comment, but we all know the government has numerous times in the past conducted covert operations of questionable ethics and legality - especially in the name of national security. The methods used to "convict" someone without trial are not transparent. Should the government be so inclined, it could use the powers of the 2012 NDAA for political reasons, under the cover of counterterrorism. Similar things have happened in many countries, as well as in the US in the 20th Century (McCarthyism, Japanese internment, etc).
Even if there aren't ulterior motives involved, there's always the chance that the people in charge of finding these "terrorists" could be wrong. You need only look to the invasion of Iraq for evidence that the government doesn't always do its due diligence. That is why we need trials.