GPL only forces you to contribute back a modification you make and publish. > In a world without copyright, companies would be free to make their own modificat

GPL only forces you to contribute back a modification you make and publish. > In a world without copyright, companies would be free to make their own modifications and keep them secret, making it more or less impossible to integrate them into a cohesive whole the way they are more or less forced to do today. Private modifications that are never shared with a third party are fine with the GPL. Eg Google doesn't have to share whatever kernel they are using on their internal servers with you.