Fruits of a poisonous tree: The idea is to not give these crimes justification and vindication. To prevent people from committing crimes in order to claim they did it for the greater good: some maniac could abduct and torture people and release the findings, accepting the consequences be it prison or death. But in their mind they are justified because their findings would help people eventually. If you don't accept the findings, then the criminal has no justification because no, there is no greater good. you just committed crimes.

similar to the right to privacy preventing police from using evidence from unlawful searches. you found a body, but you didn't have proper reasoning for the search so now the body doesn't exist as far as the courts are concerned. because otherwise the police would violate people's privacy all the time and eventually justify it by finding something.