Really, you are reacting like a concerned mom SHOULD react, but this is a good time to have her do a reality check... ask what she would have done if the surfing had gone bad.
(They don't usually, but it's a good thing for her to think about it). Ask her if she had gotten totally separated from whomever drove her to the show, what would she have done? Ask her if she thought she was making the best choices. Tell her you love her and you don't want anything crazy to happen to her. Tell her that you respect her wanting to have fun, but you also expect her to make the best choices while considering safety. And she will probably say "oh mom, don't be so old fashioned," but then she may think twice next time.
NO WAY is 14 too young to go to the show!! OMG, I would have loved to have been there myself! NIN are terrific!