[jokethrowaway]

I never understood why that's the case. Is it not to encourage unlawful activity?

Stealing an example from mr robot, if I break into someone's computer and find evidence that proves they are trafficking underage photos, I don't see why that proof should be ignored , in any circumstances.

Sure, charge me as well with breaking into someone's computer, if you can catch me, but don't dismiss my evidence.

[kian]

It is to discourage unlawful activity on the part of law enforcement.