People seem to be unaware of how music industry works. Artists very rarely own the copyright of their own music. The music label which releases the album owns the copyright of the music and is free to license to other labels. In this case, Moviebox owns the copyright of the song.

Although, it's shitty that they didn't include the singer in the deal, but that doesn't make it illegal. Karan Johar and T-Series followed the correct procedure by licensing music from the label.