According to the article above from the op, Congress wanted the no-trial detention and President Obama only signed after lawmakers removed it.
"The administration also pushed Congress to change a provision that would have denied U.S. citizens suspected of terrorism the right to trial and could have subjected them to indefinite detention. Lawmakers eventually dropped the military custody requirement for U.S. citizens or lawful U.S. residents.
"My administration will not authorize the indefinite military detention without trial of American citizens," Obama said in the signing statement. "Indeed, I believe that doing so would break with our most important traditions and values as a nation.""
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