If a government can intern suspects without trial, then it could theoretically imprison its entire populace for no reason except for its own whim. When you consider this, it is clear that governments have no right to intern people without trial. Everyone has a right to defend himself or herself from accusations and the threat of imprisonment.
One of the fundamental aspects of American law is that defendants are innocent until proven guilty. This must be proven through trial, and within this trial, the jury must be comprised of neutral individuals to avoid any bias. To intern a suspect without trial violates this right, as well as constitutional rights availed to all citizens.