Gov. Jerry Brown of California dropped the ball on Sunday when he vetoed the Trust Act, a bill aimed at keeping harmless immigrants out of the deportation dragnet — not out of misguided compassion, but to bolster public safety. The police in immigrant communities depend on the cooperation of witnesses and victims; when local officers become federal immigration deputies, fear overrides trust and crime festers. Mr. Brown’s late-night veto bitterly disappointed immigrant advocates, law-enforcement officials, religious leaders and others who had urged him to take a stand against pressure from Immigration and Customs Enforcement, which is deporting people at a blistering pace and wants to keep doing so. The agency’s Se Communities program has turned local police into handmaidens of federal immigration policy; people arrested are automatically subjected to immigration checks, leading to the deportations of tens of thousands who have no criminal records or are only minor offenders.