The SCotUS opinion in Coffin v US stated the following:

  "The principle that there is a presumption of innocence in favor of the
  accused is the undoubted law, axiomatic and elementary, and its enforcement
  lies at the foundation of the administration of our criminal law.
  ...the exclusion of an important element of proof can be justified by
  correctly instructing as to the proof admitted. The evolution of the
  principle of the presumption of innocence, and its resultant, the doctrine
  of reasonable doubt, make more apparent the correctness of these views, and
  indicate the necessity of enforcing the one in order that the other may
  continue to exist."

They can also get away with doing the exact opposite in their practice, only to be overturned at great personal expense to individuals with few resources.