I could understand if you called it unfair listening, but not unfair dealing. As Jes said, the Cubs gained no competitive advantage by having Dempster listen in besides maybe having him agree to a trade with a different team more easily. I guess you could say the Dodgers lost out on a potential boost to the team by missing out on Dempster, but I'll maintain that the Cubs wouldn't have given him away anyway. There's always the pick in next years draft they could have chased.
I could also understand if you said it entered some gray areas ethically, but i still wouldn't say it was a complete breach of ethics or crossed any line, unless the parties were identified, and he remained silent. Even if this were so and it was a breach of ethics, this was a very unique situation, and I think other gm's would recognize it as such, and not hold it against the Cubs front office. They have no history of being ethically challenged or having unfair dealings with other gm's.
In addition, every other GM wants to do whats best to improve his respective club, and if that includes dealing with Hoyer or Theo, then they will do so. Except for maybe the Dodgers, their plan to fleece the Cubs was thwarted and I could see holding a grudge. I will also admit my view may be somewhat biased against the Dodgers in this matter because they were trying to take advantage of my beloved Cubs, but oh well. Here's hoping the Dodgers miss out on the playoffs because they didn't trade for Dempster.