I think their argument is that you no longer maintain control of the object.

If you share a house with someone, and leave your drug paraphernalia in the common area, if your housemate invites the police in and they see it there they can use it in court and leverage that to search your room.

I disagree with their argument, but they are suggesting that the cloud provider is your housemate and you gave up control when you uploaded it to their common area.