Contracts and similar legal documents are weakly copyrightable. Copyright protects creative expression. Legal documents are usually not a creative but very technical expression of their content, using specific phrases that have become customary in that context. It is often not reasonably possible to phrase a legal document differently if it is supposed to convey the same meaning.
The GPL is an obvious exception because it also contains a preamble, consisting of prose text. Stripped from the preamble, the terms alone might no longer be copyrightable.
The WTFPL also contains a copyright line and meta-license, but I think the license's author may be overstating their rights there because the license contains minimal creative expression.
As I understand it, this leads to the following results:
- A license may or may not be protected by copyright.
- If it is copyright-protected, open source licenses come with a strong implied license that they may be copied freely.
- Nearly all open source licenses mandate that a copies of the licensed work are accompanied by a copy of the license, which is implies some permission to use the license document.
- So yes, licenses can absolutely have self-referential terms.
- License names may be protected by trademarks, which generally prevents a changed license from using a confusable name. For example, Apache is registered trademark.