The point is, what do you expect? Managers are turned on by fans and sacked by clubs at the drop of a hat. Every manager that has ever moved clubs has always returned to have a look at the players he rated and trusted. Since when has there been some kind of moral code that this shouldn't happen...
No, not at all. Even managers leaving with the blessing of their clubs like Martinez and Moyes go back to try to sign players they trust and have worked with consistently. You can only work and give 100% commitment to your current employer. Managers are cut down at the drop of a hat these days...