I believe BC did something himself to that phone. Either on purpose or by accident, but I believe the phone was compromised in some way even before McDreamy started inputting the wrong password, as instructed by the AT&T rep. I do not see any reason CPD would have wanted that phone to not give them any evidence. They wanted the info from that phone for their investigation. I think it was a f*up that it got wiped...perhaps helped by one BC.
Further, as of the day of arrest, CPD didn't have any info from NC's phone, so that phone didn't appear to factor into the decision to move forward with an arrest and indictment either.