Erm, because detaining people without trial is illegal.
“Oh by the way we’re gonna arrest you and hold you for 3 years in Cuba”
“On what charges?”
“Sorry old boy, top secret”
“Well, can my lawyer know?”
“What’s a lawyer? Goodbye. Oh, while we’re at it we’re gonna dress you up like Slipknot (Metal band who wear boiler suits and masks- for the oldies amongst you) and mentally torture you. Have fun”
All for (possibly) being in the wrong place at the wrong time.
Sure glad it wasn’t me, especially considering Abu Gharib