With fingerprint unlock it's not your choice. That's what makes it clever. The cop can grab your finger and hold it to the scanner while you tell him not to do that.
Presumably you must disclose that "doing that" would wipe the device... and you better hope this is caught on some neutral party's camera so there is a record of you saying that.
False presumption. You are just making that up, right?
I am not a lawyer, I am just "making up" what exculpatory evidence I would prefer to have on my side when potentially confronted with a charge of destruction of evidence after pulling a stunt like this.