I can understand the safety concern - and I think the decision to turn around was ultimately the right call. Especially given that they had called for people turning off BT for some time.
The fact that the device was not turned off suggests to me that the owner did not know they were the cause of this. If they had done this by intent and were set on going through with it even after the turnaround was initiated, they would have also had the sense to drop the device into some other seat or leave it in the lavatory...
If it turns out they did this with full intent, there should be some _appropriate_ consequence