While I'm no legal scholar, I do question the legality of that practice as well. In general, society tries to encourage respect for LE officers, and their hiring practices generally include some type of background check regarding their personal integrity. If the expectation is for them to be generally honest people, shouldn't their obtaining of information through subterfuge be considered an unreasonable search?
Having said that, I recognize the need for undercover operations/investigations, but would suggest that they be conducted under the limited sanctions of a warrant.
SCOTUS says .... Holland v. McGinnis among others.