The Supreme Court decided in Miranda v. Arizona that people had to be told their rights under the 5th and 6th Amendments, since if people don't know that they have those rights, they don't really protect them. Those rights, of course, can be waived, as can the right not to be searched without a warrant.
If a police officer comes to your house without a warrant, says that they think you have illegal drugs, and asks if they can enter to search the property, do they have to disclose that it isn't legally required since there isn't a warrant? A reasonable person may feel as though they don't have a choice.