The analogy is fatally flawed because, as GP points out, it's trivial to copy illicit material onto the medium in question. You can't copy blood, and I don't see how you can find and plant blood of a known murder victim.
I don't think that's really a fatal flaw. I mean, the mechanic could have killed someone, put the body in the trunk of your car while it was in the shop, and then disposed of the body elsewhere. The legal system is designed to handle cases that aren't exactly "open and shut" and there's a balance to be had between protecting against possibly tainted evidence and being able to bring criminals to justice.
Just copying child porn onto somebody's computer shouldn't be enough to convict them, and if its is, then the problem is with the legal system elsewhere, not with the system of mandatory reporters.
Just copying child porn onto somebody's computer shouldn't be enough to convict them, and if its is, then the problem is with the legal system elsewhere, not with the system of mandatory reporters.