Return ticket needed at Abu Dhabi Pre-clearance ?

[nivas01]

My in laws are traveling to US via Abu Dhabi(B2 visa) and I have booked only one way ticket for them. I read somewhere that they also would need to have return ticket with them during the pre-immigration clearance at abu dhabi .
Is this true ? Can somebody please confirm ?
I tried to contact CBP on this , but couldnt reach them.