What the heck are you talking about? It's not like he is leaving Ferrari to join the competition, they let go of him 3 years ago, they forced him out a year earlier than he wanted to retire just to secure Kimi.
Do you think that Michael Jordan coming out of retirement to play for the Wizzards was unethical? Ferrari did not want Schumi 3 years ago, he is free to race for anyone he wants. I don't think this has anything to do with money either, he is borth Billions of Euros already