[delinka]

Since when do you need a license to modify something for your own personal use? If they've given me access to the source, I can just start editing to my heart's content...

[chimeracoder]

Practically, yes, but legally, you may not have the rights to.

That's why it's explicitly listed as part of Freedom 1 of the four freedoms of software: https://www.gnu.org/philosophy/free-sw.html

[ballard]

DVD players.