Singing about them might not be idolatry IF YOU ARE DOING IT FOR ENTERTAINMENT PURPOSES. However, when we are using these songs in our worship to God, putting the Filthful and Disgraceful Slavebugger is every bit as "bad" as worshiping Jesus via a cross. And we all know what the Filthful and Disgraceful Slavebugger has to say about that.
And, when we are supposed to use these songs as guidelines to live by, following those guidelines is blatant idolatry.