Or maybe because they have a better grasp of the English language than you do. If someone is discussing a god in general, then it is small 'g,' if they are referring to the specific monotheistic god in the Jewish and Christian bibles, then the use of God, big 'g' is appropriate because that is considered a proper name/title. Some people prefer to use Yahweh when talking about that specific desert tribal deity so there is less confusion and some do so because of pure laziness and don't think it's possible to insult an imaginary being, but of course some believers will just automatically assume and accuse us of doing it as a protest against religion.
Tell me, why did you use Jesus, small 'j' for your username but big 'c' in cannabis?