Or just singing a little bit of that song because it's popular now.
Yep. image

And I think it also has to do wish people being all, "OMG Justin needs to move on" everytime he adds a twist to CMAR. I would imagine that he is just changing a song up that he sings all the time. It doesn't have to be about being angry at Britney. it could be about taking something that happened in his life and putting a new twist on it.
I agree. This is nothing new and yet, some people still feel the need to get upset about it. It's become as much a part of the concert performances of CMAR as the original lyrics and probably always will be.