Fruit of the Poison Tree Scam

Opinion writers at the Wall Street Journal have invoked the legal doctrine known as the "fruit of the poisonous tree" to discredit the Mueller investigation. They argue that, because an FBI agent who was in on the initiation of a probe into collusion between Russia and the Trump campaign has been shown to have had a strong aversion to Donald Trump, the entire investigation, including the current version headed by Special Counsel Robert Mueller, is tainted and any findings made or actions taken will be inadmissible and of no legal weight.
Patently, those editorial writers either failed to consult knowledgeable lawyers or received very bad legal advice. The fruit of the poisonous tree doctrine was created by courts to prevent and deter police and prosecutors from using evidence that would not have been discovered but for a seminal violation of an offender's constitutional rights.
Even if the investigation into Trump-Russia collusion originated from improper motives, a highly doubtful claim, the subsequent discovery of incriminating evidence by that investigation is, nevertheless, admissible provided it was not illegally obtained, i.e. did not flow from a violation of the defendant's Fourth Amendment rights.
Since the argument has no legal merit, it would seem to be part of an orchestrated campaign to politically undermine any findings that may be adverse to the Trump campaign.