WP uses your live account for everything they can, which does streamline things, but also does limits things a bit when the way we use things do not fit they way that they expected we would use them. I don't know of a way to set the live account on the WP SmartGlass app. SmartGlass assumes the XBox live account is the live account on your WP, which is the case most times but not for a family member who is also using your account. Microsoft assumes only one person uses that XBox live account, which makes sense from a GamerTag / on-line gaming perspective, but with all of the other entertainment options, more and more people are turning on the XBox and logging into an account just to watch a movie or listen to some music. XBox live has change quite a bit since it first started.
Android is different, since it wants a google account for everything google can, so the SmartGlass app has to ask for your XBox live account.