The U.S. supreme courts, and federal courts have generally refused to side with sex offenders under most circumstances. See:

Connecticut Dept. of Public Safety v. Doe

Patterson v. State

Etc, etc, etc. If actually took a day and made a list, it would be three pages long, but I’m not going to flood George’s blog with that. Nor do I really want to spend that much time using LEXIS/NEXIS. The point is that with such a heinous crime, even the judiciary is reluctant to create loopholes for the child molesters to potentially escape through.
Anyways, the protocol and case law for I-STULE(Internet Solicitations to Undercover Law Enforcment. – This includes people working as agents of law enforcement.) The courts have repeatadly held that officers must follow the case law and general police procedures pertaining to undercover drug operations in conducting the sting.
As for the “violation of the presumption of innocense” I think you are confusing “presumption of innocense” with “burden of proof”. The courts routinely require various parties, including defendants to meet certain “burdens of proof” regarding there case. A defendant in a bail hearing must prove that she/he isn’t a flight risk in order to acquire bail. The burden of proof also falls on the defendant likewise to prove they knew that they were chatting to was an officer/ agent of the police. I’m going to go out on a limb here, and say that this probably almost never happens, because in a typical sting, an arrest isn’t made until the offender meets the “child.” If they knew it was the police, then they wouldn’t have come, would they? It would be a very, very difficult defense to pull off.