I get what your saying, I don't understand why some of these bands present themselves the way they do but we don't know what happens behind the scenes at these festivals (for eg.) so yeah, I just leave it that hoping that they would have a good testimony to other bands as to why they aren't indulging in alcohol, drugs, cursing and having sex with loose women.
i don't doubt that there are charlatans who are called "Christian bands" but i think the attitude & justification of those bands that tour with secular bands or play for secular crowds has something to do with this:
And as he reclined at table in his house, many tax collectors and sinners were reclining with Jesus and his disciples, for there were many who followed him. And the scribes of the Pharisees, when they saw that he was eating with sinners and tax collectors, said to his disciples, “Why does he eat with tax collectors and sinners?” And when Jesus heard it, he said to them, “Those who are well have no need of a physician, but those who are sick. I came not to call the righteous, but sinners.”
(Mark 2:13-17)
being "called out of the world" is not the same thing as being called to complete isolation. like you say, we don't know exactly what goes on behind the scenes, but we can hope that the Lord uses them as a testimony to the world, and i think for a lot of Christian musicians this sort of evangelism is the ministry that they feel called to. we don't send missionaries to the church across the street; we send them to people that haven't heard or accepted the gospel.