Under your theory anyone who advertises an illegal activity can put a "no law enforcement allowed" which then would require the police to obtain a warrant to enter the premises to arrest the person committing the illegal activity. You would be right if when the police (or undercover) is not allowed into the location, and thus no allowed to commit a crime or witness criminal activity inside the location. But if the person committing the crime allows the police or undercover to enter and commits a crime they can be arrested right then and will not have a viable argument in regards to a warrantless arrest argument. Clearly no violation of the 4th Amendment.
Your entrapment argument makes more sense. It will be defeated, however 90% of the time as it is fact dependent and relies on the totality of the circumstances. Typically the cops have to make a brazen mistake in order for you to prevail on this argument.