There are two likely ways I could interpret your parent regarding crime. One is that they worry that if only criminals use the tool, the tool will be targeted

There are two likely ways I could interpret your parent regarding crime. One is that they worry that if only criminals use the tool, the tool will be targeted by legislators and simple possession/ usage of the tool will eventually be criminalized. The other is that there needs to be a crowd of innocent people to hide the guilty person and make the investigators / prosecutors work hard to find other corroborating evidence of the crime (not just using the channel of comms). A third that has nothing to do with crime is that all people deserve to have strong privacy and we should normalize using a very secure method of comms.