Does this stronger "fruit of the poisonous tree" doctrine of the US does apply for all those three letter agencies in your opinion? Or just for the RSI?
While conducting these clandestine operations they may also collect information which could convict someone domestically in court... but US courts will not permit that evidence to be submitted as it was collected illegally.
External intelligence agencies would be kind of an exemption. But not in the case of the CIA, as they give information to internal intelligence agencies, although they shouldn't, at least by the law.