The file in question has a peculier paragraph with the following contents:
Although Linux is supplied with the complete source code, it is copyrighted software, not public domain. However, it is available for free under the GNU Public License. See the GPL for more information. The programs that run under Linux have each their own copyright, although much of it uses the GPL as well. All of the software on the FTP site is freely distributable (or else it shouldn't be there)
It says although Linux is available for free, but still its source code is copyrighted "not public domain"
What does it mean by saying that its copyrighted by still available for free, the file specifically says that the source is not under public domain, so is it against the law to modify linux kernel?