Do you not see the problem with allowing them to convict people based on illegal investigations? If they are allowed to use illegal methods and still get a conviction, they will continue to do so. Cases should be thrown out when the investigation was done illegally, as a way to strongly ensure such things do not happen. A guilty person going free is bad, but far worse is a rogue police force with no regard for the law and the good and proper limits placed upon them to prevent abuse.