Trump campaign has the right to discuss its policy positions with whomever cares to listen. That's absolutely not against the law. I'll point out that Democrat politicians campaigning for election have also been abroad and talked with others. This is nothing unique to the Trump campaign.
But Deep Staters including at FBI hate the policy positions of the Trump campaign, and want to get rid of Trump & co by any means necessary, whether fair or foul, whether by hook or by crook.
So they eavesdropped on Flynn under false pretexts, and then even when nothing turned up wrong (as confirmed by FBI Jan-4-2017 memo), they still tried to question Flynn as a last-minute gambit to screw him, hoping they could at least get him fired.
Deep State wants its own policy agenda to prevail no matter what, and this was their attempt at a coup d'etat. They felt that if they could nail Flynn, then they could parlay that into an expanding investigation that would allow them to nail President Trump and get rid of him too.
Coup d'etat is a fundamental breach of the constitution, and is treason.
I hope these bastards are charged with treason, even if they face the death penalty.
Anyone who wins an election should be able to put in their preferred foreign policy priorities. Nobody at FBI gets a veto over US elections. If you're at FBI and somebody gets elected who you don't like, then tough luck - you don't get to inflict a coup d'etat - that's treason.