Neither of them are country and neither are their songs, yet they were allowed to perform at the ceremony, which took place last night in Nashville, Tennessee.
Ariana's costume illuminated the stage while she performed with Little Big Town. Producers clearly wanted to deliver some of that commercial appeal to the show despite the randomness. It would be like Coldplay performing their music at the BET or Soul Train Awards. It just wouldn't make sense. I blame Scooter Braun for being such an amazing manager.
Ariana's part starts around the 3 minute mark.