Answer #1. You have to prove the espionage firm did something illegal. This is sometimes much harder than it might seem.
I was once interviewed by an IT manager of a major telecoms company. They had security like nothing I had ever seen - it was like the introduction of the old spy comedy Get Smart - layer after layer of heavy doors, big muscle doormen, ID checks, cameras…
I asked why they had all the security. The IT manager said “our main rival is hiring investigators to learn anything about us, any way they can”. Of course, his firm was doing the same to the rival firm - so they were in no position to complain about illegal tactics.
And of course, if the other firm had snuck someone in, someone who planted say a radio network bug, to give the spy direct access to the firm’s internal network - how could anyone prove who they were, and why they were there? I’m sure that “copping a trespass charge” was part of the deal for spies who entered the premises illegally.
Answer #2. Simple : Spying is not limited to hacking. more