This is my core question as well. At what point do you have to maintain property so the government can use it to testify against yourself?
If I have a dash-cam, and I wipe the SD card, can the government imply that because I erased the card, it must prove that I was speeding? The dash-cam automatically over-writes old footage - perpetually destroying evidence.
Given nebulous cases such as "hacking" a site by looking at the HTML[0], am I destroying evidence of crimes whenever I format my PC? I hope the government requires specific charges and more proof of a crime other than missing evidence. Say I destroy my diary - can the government claim that is the key evidence where I confessed to being the gunman on the grassy knoll?