You are right in the fact that most constitutions, and probably that of the USA too, comprise some kind of exceptional procedures allowing the government to override the rule of law in the case of an emergency. I think that they're required in order to deal with those cases such as angry people with pitchforks burning down cities etc, something that still happened once in a while in the past century, but I don't expect those procedures to have been applied often nowadays.
And I don't think people have the *right* to lie: having a right to something doesn't mean having the permission to do that thing, it means that there's some law stating explicitly that that something must be given to those who haven't got it.
Anyway, I was wrong in my post above: there appears to be no explicit law against entrapment, if I understand correctly it's just a matter of interpretation by the courts, which has oscillated over the course of years.
"Don't try to outweird me, three-eyes. I get stranger things than you free with my breakfast cereal." - Zaphod Beeblebrox in "Hithiker's Guide to the Galaxy"