Just because you can view the source code, doesn't mean you're allowed to modify it and then use the modified version for yourself or publish the modified versi

Just because you can view the source code, doesn't mean you're allowed to modify it and then use the modified version for yourself or publish the modified version. GPL ensures that you're allowed these things and that anyone, who does modify it, has to then again allow these things to anyone who uses their version. Yes, a good chunk of GPL – that the source code has to be published along with the software, is pointless here, so a simpler license could have been chosen, but I guess, they went with GPL anyways, as it's a proven license which has already had a few courts look over it.