1) Perhaps they don’t want to trigger a different licensing model (on either the music or the lyrics) that applies specifically to karaoke devices and venues. Just a guess, not a music lawyer, but all sorts of unexpected terms apply to music licensing (such as streaming TV services not being able to use music from the original TV broadcast) 2) Perhaps they don’t want to associate with the karaoke “brand” which some users might perceive as kitschy or low-quality (have you ever seen a karaoke video?)