You can still modify it for my purposes so long all users who interacts with it over a network has access to the modified source code. If the number of users are you then modify away!
> Usage is whatever you do with it that's not distribution in the sense defined by the copyright law.
That is the tricky part. A video streaming site is considered distribution by many copyright laws. There was a time where simply using a website was not considered distribution, but then scope of copyright was extended and concepts like "public performance" and "making available to the public" was applied to works provided through web services.
GPLv3 however gives an additional permission that allow "interaction with a user through a computer network", even if copyright law would forbid it.
Personally I agree with copyleft licenses for as long as they are just restricting distribution as defined by copyright law. This because IMO there needs to be a clear, lawful boundary for what FOSS licenses can and cannot restrict.
> Usage is whatever you do with it that's not distribution in the sense defined by the copyright law.
That is the tricky part. A video streaming site is considered distribution by many copyright laws. There was a time where simply using a website was not considered distribution, but then scope of copyright was extended and concepts like "public performance" and "making available to the public" was applied to works provided through web services.
GPLv3 however gives an additional permission that allow "interaction with a user through a computer network", even if copyright law would forbid it.