He's only singing parts that are already covered by the backing track, and as I said, he's even lip syncing to parts he isn't capable of singing with his range (I love his vocals, but he's not JLB!).
Just because someone with a mid-baritone range is singing along to a part where a high tenor is the lead, for example, doesn't mean the mid-baritone singer is somehow trying to look like he is faking the hard part. He isn't. He's singing a lower harmony that makes the higher lead part sound better, even if the lower harmony is so far in the background that most don't consciously notice it.
Sure, but it isn't so far in the background that most don't consciously notice it. It's completely inaudible. So inaudible that we're having an argument about whether it even exists.
Here's an example. One of the JP sing-along moments that really stuck out to me was the chorus of War Inside My Head, where he goes up to the mic to sing along to the "saying things never said" line (which is, of course, itself a JLB recording). There are three options here:
1) JP is singing an awesome harmony to JLB's recording of the backing vocal. This is unlikely, given that 1) the awesome harmony is 100% inaudible in the recording and 2) that line has never been performed with a harmony, so it's unlikely that one had been added for this show. DT tends to remove harmonies when they do their live performances, not add them.
2) JP is doubling up the melody to JLB's recording of the backing vocal. This is also unlikely, given that 1) again, JP's vocals are completely inaudible if they're there at all, and 2) DT tends not to do that in their performances either. In the past, JP and MP tended only to sing if there was a harmony for them to sing (e.g. I Walk Beside You, Through Her Eyes) or if there was a second melody line (e.g. Constant Motion, The Dark Eternal Night). It was never important to JP to double up on JLB's melody lines in the past, though it stands to reason that if that
were important to him, he would, you know, make it so that you can hear his vocals. So that leaves us with option 3:
3) JP is singing or mouthing along to JLB's vocal track, trying to make it seem as if the backing track is coming out of his mouth. This isn't protocol for DT but maybe JP thought that would be better for the "concert experience" of the live release?
Basically, you can choose to believe that JP is singing an awesome melody/harmony that is impossible to hear for some dubious reason, but what evidence is there for that? The lip-synching solution is much simpler.