Because the transporter replica is not you. That is a separate person with their own separate consciousness. You are murdered and someone else is born.
Murder is a crime as decided by humans. There are circumstances where killing another person is not a crime, because we have agreed it is not a crime. A doctor turning off life support can be fully legal even though it results in the destruction of a human life. There is no ultimate principle at work. The law is a set of rules that we have (implicitly) agreed to.
Likewise, we as a society could decide that a person has all of their rights transferred to their replica as soon as they walked into a transporter.
Uh, obviously?