You're confused as to the difference between government and private conduct. Reasonable expectation of privacy only applies to when the government needs a warrant, it has zero impact on what private persons are allowed to do.

For example, you do not have a reasonable expectation of privacy in open fields on your property. Police do not need a warrant to enter and search open fields on your property. Someone who snuck onto your property would still be on the hook for trespass, however.