Why aren't lisp keywords protected?
Because they aren't keywords, they are just library functions like any other.
Where can this be useful,
It means that you, as a user, have the same power as the language designer. Making a change to a language can take several decades and is a massive undertaking. Writing a new library function only takes seconds and can be done by anyone.
AFAIK, all programming languages have reserved keywords.
No, not all. Lisp doesn't, for example. In fact, Lisp doesn't really have syntax at all.
Io has no keywords, neither have Ioke or Seph. Poison and Potion also don't have keywords.
Smalltalk also has only a very small number of keywords:
thisContext. Newspeak has almost the same list (
outer). In both cases,
false could just as easily be method calls instead, leaving only 3 keywords. (Actually, Gilad Bracha, the designer of Newspeak said that all of them could be made method calls, but it wouldn't be worth it.)