When I looked up detainment, it said "In the US, police can detain someone for a short period based on reasonable suspicion of criminal activity or if they have probable cause to believe a crime has been committed." The CNN crew were not doing anything criminal & there was no probable cause .
That is indeed one situation where a detainment can be made, but it’s also not the only situation. It can also be done for anyone whose safety an officer feels may be in jeopardy. This is done often during domestic disputes where all parties are detained while an officer assesses the situation.