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.
It is a good thing then that there is no way to overcome this by storing the encryption key in such a way that it will be destroyed (or at least plausibly destroyed) in the act of seizing it.
Implementing a dead man switch for an encryption key is much easier then doing the same for volumes of data.