The only reason I can think of is the FBI did not want to tip the husband since they were not ready to arrest him.
If she was indeed detained, searched and interviewed at the airport - Awan quite understandably would already have been “tipped off” by that alone.
I think L/E let the wife go b/c they needed to know who these characters are connected to over in Pakistan.
She is probably under constant surveillance.
The Pakis also wire-transferred money to two individuals in Pakistan who most probably flew the coop once they had the money.
L/E needs to know the whereabouts of these two individuals.