A great deal of the traditional antipathy to snitching is less toward the act than the actor. The snitch is personally nearly always an unpleasant character, currying favor with the authorities, whether it’s the Mafia snitch bargaining for immunity, the institutional snitch seeking glory in the newspapers, or the fourth-grade snitch after a little gold star. Still, this is not the heart of the matter.
Snitching involves a conflict of loyalties to the abstract — an external code of morality, or an institution — and the personal — your friends, gang, colleagues, classmates. (Or, to put it another way, to the larger group and the smaller.) The weaker the commitment to the abstract norm, the stronger the taboo against snitching. It is strongest of all in criminals, who have almost no faith in the abstract code and for whom the conflict therefore scarcely exists. Only if the behavior harms the gang itself, like skimming off the proceeds, does omerta go by the boards.
By the same token, the stronger the loyalty to the smaller group, the stronger the taboo against snitching. As one of Davies’ commenters points out, it goes harder for whistleblowers in Europe, where the bureaucracy is regarded as the Godhead, than in America, which is rather less reverential.
In some special cases of snitching there is also a knowledge problem, and moral philosophers tend not to interest themselves in knowledge problems. You don’t tell your best friend’s wife that he has been cheating on her partly because no one outside has been significantly hurt — the abstract loyalty is close to nil — but mostly because you don’t know enough about the complex, imperfect structure that is their marriage to disturb it. A man’s got to know his limitations.
All of this is less a topic for philosophy than literature, which has had more to say on the subject. The snitch-lit classic is Ibsen’s An Enemy of the People, an unregenerate celebration of the snitch. While in Ibsen’s play no one supports Dr. Stockmann except one friend and his family (and his wife only dubiously), in the American remake, Jaws (props to Ian Hamet for pointing this out), Mayor Vaughn, the town’s alleged representative, has the support of almost no one. America attenuates local loyalties, which attenuates the taboo against snitching.
(Note: Last paragraph rewritten, after rereading An Enemy of the People. The moral is not to discuss books you haven’t read in fifteen years.)