There are stark legal differences between arrest and detention. TSA agents are authorized to detain, but not arrest, suspects. The bar for detention is lower, and detention may be performed by people other than sworn peace officers. Even private security officers - such as those at casinos in Nevada - are legally permitted to detain people for the purpose of notifying the police if they believe a felony has occurred on the property. NRS 171.1235
By contrast, an arrest may only be performed by a peace officer with probable cause to believe a crime has occurred. Detained suspects do not have to have their rights read to them, whereas they must be read to arrested individuals. Detained suspects are either arrested or released usually within hours, as was the case here.
Detention and arrest are vastly different things, both legally and practically, that are used for different purposes. The system worked here: he walked into an airport displaying many red flags. He was detained for a few hours, determined not to be a threat, and was released.