If you want to redistribute and/or modify TeX Live, please see the copying conditions.
As a general statement, the TeX Live maintainers agree (at least within the shared purpose of working on TeX Live) with the general principles and philosophy of the free software movement.
Therefore, to the best of our knowledge, all the software in TeX Live meets the requirements of the Free Software Foundation's definition of free software, and the Debian Free Software Guidelines. In the rare cases of conflict, we generally follow the FSF. Furthermore, the material in TeX Live should not require nonfree software to be useful.
In short, this means that all the material in TeX Live may be freely used, copied, modified, and/or redistributed, subject to (in many cases) the sources remaining freely available. (Note: this statement is not true of all the software in the CTAN snapshot or in proTeXt, which are distributed alongside TeX Live in the TeX Collection.)
Of course, you must not yourself claim copyright (especially with a proprietary license) on TeX Live just because you redistribute it. Again, see the copying conditions for more information.
Donald Knuth released his original TeX and Metafont programs, and Computer Modern fonts, to the public domain: TeX copyright page, MF copyright page, CM copyright page, article by DEK.