The code maintainer (or owner) makes the code available to the world without asking for anything in return, but if you send him a patch he is somehow obligated to implement it? (I'm not saying this is what you're implying)
Open Source goes both ways: nobody forces you to use some rando's code off of github but also nobody forces said rando to to use some rando^2's patch for his own code
Yeah, nobody is forced to do anything, but humans are weird social creatures and feel pressure to do certain things even when not externally forced. If people were just doing a robotic cost/benefit trade off then this wouldn't really be an issue, instead both sides get a bit emotionally invested in it.
Open Source goes both ways: nobody forces you to use some rando's code off of github but also nobody forces said rando to to use some rando^2's patch for his own code