Just a guess, but if I remember correctly they weren't allowed to bring their own equiptment. They had to share this cheap, "communal" gear with other bands that were playing on that stage (I only recall Opeth using the same gear, which included very cheap looking strats and generic Marshall amps, as I didn't watch any other bands on that insulting stage). It probably didn't feel very comfortable to them.
Also, Jonas was singing off key, practically the entire time, which leads me to believe that he was performing without a working monitor.