> If the prosection doesn't have to demonstrate that something was valid evidence beyond a shadow of doubt that's rife for all sorts of abuse.

Assuming that is the preceding paragraph you reference, then there is nothing to respond to because the law is clear. As zugi mentions, obstruction of a federal investigation by tampering with evidence (i.e. potentially anything in the scope of a particular search of which you are aware) in just about any sort of way is illegal under 18 USC § 1519 (and again, most states have similar laws with regard to state officials/departments and searches).

This subthread, the general thread, the linked article, and the original event it covers are all within the context of someone aware they are detained or under investigation. In that context, the prosecution doesn't need to demonstrate that what was destroyed is evidence. Just that the suspect obstructed the search by destroying, altering, or tampering with something in the search's lawful scope.

This state of affairs isn't any more or less rife for abuse than the general process for obtaining a warrant or the standards for an officer assessing probably cause.