Most of the opposition I have seen, has not really been the people opposing it, but people complaining that someone opposed it. For that reason I'd guess it was parents, primarily, who made the complaints.
If parents it can be any number of reasons. Extra noises in the house, tense noises. Kids not being able to hear when spoken to. Worries about early onset of tinnitus, which never repairs. And just wanting a more peaceful household at the end of a long day.
But what you're likely asking, is why Christians hold to such a rigid standard for their lives, and expect others to follow it. Not every Christian behaves this way, or believes this way, or avoids the same things.
I know Christians who avoid pork but not alcohol. Christians who play in metal bands but don't own a TV. Ones who dress provocatively but lead worship. Everyone has standards in their heads, of what they believe they should be doing.
Ultimately, we claim to live for God and carry out His purposes, living in the ways of His kingdom. But not all of us can see how His kingdom operates. Paul writes about living like athletes in training -- abstaining from anything that will slow us down in the race. That can be different for each person.
But if there are obvious things, like putting lyrics in your head that oppose your own beliefs, then we are just fooling ourselves by thinking we won't be impacted. Defending it can be a clue that we already have been impacted. If we were opposed to machine gun shootings in schools, would we play a game where we put ourselves in the position of shooting?
It's about living our lives honestly, openly, being real with others. Speaking out when we feel it should be done, and allowing others to speak out in too, even if it's in opposition.
If we think something will ultimately harm people, then there are times that it should be voiced. Not dictated, but expressed.
And that's not so much different from metal music -- where people want to shout out about injustice and inequity. Everyone wants to be able to say what they feel.