Apparently he has several phones, so although it is irresponsible (considering his position), it may have been overlooked easier than if he carried only one.
Must have had something he wanted to hide on that particular cell phone.
Given he was going back and forth frequently between Europe and the US, he might well use one phone here and another over there. That's what I do. I have a GSM phone that I use in UK. I just buy a pay-go and activate when I'm there. Dirt cheap compared to roaming.