This is utterly false and makes me suspect you really don't understand either common law or the US constitution.
I understand both, having been in court many times and served also as both a lay and expert witness, and having helped many attorneys prepare cases that lie within my professional field of expertise. You're harping on abstract concepts of high-school civics while assiduously ignoring the mechanics of what actually happens in a court room and how evidence is actually examined and tested.
Perhaps, in undermining the prosecutions claims the defence may suggest innocent explanations for their evidence, but they are under no obligation to prove any of their claims...
And the jury is under no obligation to believe a defense's vague handwaving defense.
...just establish a reasonable doubt about the prosecutions claims. But even this is not necessary, all that is required is that the prosecution evidence is undermined, doubt cast upon their witnesses, their expert testimony contradicted.
And somehow you think this can all be done without proposing an alternative explanation. How can you "cast doubt" upon a witness except by showing that the witness didn't actually see what she says she saw, that she isn't as sure of it as it first appeared, or that the inconsistencies in her story suggest she may be lying? How does this not constitute an alternative explanation for the allegations in her direct testimony? The favorable counsel will present that witness as telling the whole, unaltered truth.
How do you propose to counter expert testimony except by proposing contrary testimony that conveys an impression that the other witness isn't telling the whole story or has made an error. How is this
not an alternative theory for how the first expert's allegations and judgment came to be? Or conversely, shaking faith in the foundation laid for that witness offers the alternative theory that the "expert" really isn't one, even though he was represented as one?
How do you propose to discount circumstantial and documentary evidence except by offering the possibility that it was planted, coincidence, or does not relate to the defendant or the crime? Are you simply going to let it stand and hope it's not convincing? No, you must propose an alternative theory for how the defendant's DNA, for example, is found on the murder weapon or how the second set of accounting books happened to be discovered in the defendant's desk.
I really grow tired of the rhetorical games that most JFK conspiracy theories rely on, which eschew any and all obligation to actually show a more defensible line of reasoning that those they lambast, all while accusing their critics and detractors of some presumed lack of erudition.