I don’t understand how it is legal to eavesdrop without a warrant, even if you are target bad-faith actors and have probable cause - that’s how you get warrants

I don’t understand how it is legal to eavesdrop without a warrant, even if you are target bad-faith actors and have probable cause - that’s how you get warrants in the first place. Furthermore, everyone on this thread is talking about more secure communication. But my mind always goes to assuming every communication channel is compromised by default, and then flooding it with many, many false messages and wasting adversary resources chasing them.