The same thing happened to me! My mom and I went to meet with her last year and she assured us that it was totally okay to bring in our own florist, photographer and mariachi... then i emailed her to give her all the info and she responded the same way she did to you. So Frustrating!! In a way, they're forcing you to pay more to use their vendors b/c they're marked up. I even asked if there was any room for negotiation and the response was of course, NO. ugh!
Keep me posted, i'll also keep everyone posted on what we get worked out. Our wedding is in 6 weeks and we're stressing out a little.