It's definitely a rights issue but also some artists didn't want a certain performance ever released, like Robert Plant not wanting "Innuendo" ever to be seen/heard again.
over at Queen's official site, www.queenonline.com they're selling an NTSC version of the tribute DVD! This is what it says;
"Pre-Order the Freddie Mercury Tribute Concert Special 10th Anniversary Edition NTSC Format able to play in Multi-Region machines"
Ok so what the hell does that mean...