&gt;The purpose of the exclusionary rule and the &quot;fruit of the poisonous tree&quot; is to prevent the police from purposely violating the 4th amendment. There is no other reason why illegally obtained evidence shouldn&#x27;t be used to convict a criminal.<p>Even if we accept this, allowing the police to claim ignorance gives a loop hole to allow for illegally obtaining evidence.  That is why the evidence must be obtained legally, not &#x27;I thought it was legal but it wasn&#x27;t&#x27;.