Logic,

You are applying your own culture/ideas/laws somewhere that they don't apply.

This is the law of the land, and ultimately, they got custody.

If they proceeded with the custody case first, Rebecca could have hidden the boy she calls Adam where they can never find him. What they did is just a preemptive attack.

You would be personally happy if they had asked for custody, she hid the boy, they won the custody case without the ability to get the boy.