Law enforcement officers, of all people, should know and respect The Law, shouldn't they? I mean, "ignorance of the law is no excuse", and they are doing a special job which requires special knowledge, skills and care. So, I think that we can assume they knew they weren't doing "the right thing". Stingrays are pretty clearly against the spirit of American Democracy and Law Enforcement, as traditionally held.

Beyond that, the ends rarely justify the means, even legally. There are supposed to be limits to how the police find stuff out. See 4th Amendment. Traditional sense of fair play, decency, etc.