What? Performers don’t have previous commitments? That was my first thought when I heard this. Sure enough, Lady Gaga was in the middle of an international tour which began in April 2012. On the convention night the GOP asked her to perform at, she was on stage in Helsinki, Finland. (http://en.wikipedia.org/wiki/Born_This_Way_Ball)
Much ado about nothing.
BTW, I didn’t look up Dolly Parton’s schedule. She likewise may have had a previous commitment.
If true, it certainly shows how culturally tone deaf the RNC is.