When lies of that nature are permitted by law, it throws into question the entire legal process. How is a defendant to know whether, say, the plea bargain they're being offered by the DA will in fact be honored, when the previous (false) promises made by the police weren't?

If a company representative lies and tells me the product comes with a ten year warranty when in fact it's only five years, that's fraud. You'd figure a lie with bigger consequences (jail, etc.) would be considered worse than fraud, but surprisingly not.