In that case, couldn't they just coat the finger with the same stuff they're coating the 3D printed replica with? Actually, how are they getting around the RF issue?
I suspect deformation due to decomposition. Without doing any research I'd say that the decomposition process may rapidly change finger prints enough to no longer be recognized.
Alternatively, and possibly more likely, as a murder victim there's any number of ways his fingerprints could have been obliterated.
My complete guess based is that the body is either already cremated, or in the hands of the family of the deceased. And they wouldn't exactly be jumping at the opportunity to let the police mess with their dead relative.
And they probably want something they could reliably continue to unlock the phone with. Sure, they could just remove all authentication once they're in, but maybe they want to tamper with the phone and its settings as little as possible.
I imagine there are some Frankensteinian ways to create the necessary current to activate the RF sensor, but… well, Frankenstein