Scientology is an applied religious philosophy which is to be applied exactly. If it was not copyrighted, it could be altered and republished. See What is the damage of knowing about or using this data? for more information on altering Scientology.
It is not at all uncommon for religious organizations to copyright their texts in order to protect them from alteration and misuse. For example, the Gideon Bible found in almost every hotel room in the United States carries a copyright notice. In the preface to the Revised Standard Version of the Bible, you will find the following:
"Because of unhappy experience with unauthorized publication in the two decades between 1881 and 1901, which tampered with the text of the English Revised Version in the supposed interest of the American public, the American Standard Version was copyrighted, to protect the text from unauthorized changes."
As far back as the early 1940s, bishops of the Methodist Church, acting on behalf of themselves and the Church, brought suit against certain former members who had set up a rival Church organization and were claiming the right to the property and to the use of the name of the Methodist Episcopal Church, South (the previous name of the Methodist Church prior to a union with two other Methodist Churches).
The Fourth Circuit Court of Appeals found in favor of the Methodist Church, ruling that:
"The constitutional right of men to worship God does not give them the right in doing so to make use of a name which will enable them to appropriate goodwill which has been built up by a religious society with which they are no longer connected."
And, in a 1986 case involving the First Church of Christ, Scientist, the Court of Appeals for the District of Columbia noted that
"Religious works are eligible for protection under general copyright laws, and for decades Science and Health [the central theological text of the Christian Science faith] was unproblematically the beneficiary of that security, as more than thirty editions and translations of the Bible currently are."
A pretty good article about the Church of Scientology and its copyright protection cases is on the website of the Ontario Consultants on Religious Tolerance.