They aren't stupid, they bit copy (dd) the device when it's seized. Now a local police agency might not do this but anything involving the fed's is going to be copied the second they get their hands on the data, even if it's encrypted. This is directly to prevent challenges on data integrity and to prevent dead man switches.
Ideally, whenever the phone wipes itself and destroys its copy of the master encryption key to the phone's storage, then the only way to get the data is to use brute force the 128 bit random key. Even if you have a perfect copy of the data, without the key that's locked up in the phone's security processor, your copy is useless.