See, I don't see that Disney is going to "lose" business. There will always be someone else who is ready to book and use Disney services. I don't think they are losing money when some brides (and I believe the percentage is small) use outside vendors...Disney is still making money on the dessert parties, the rooms, the wedding package itself.
Conversely, if Disney felt they were losing money because of brides using outside vendors, maybe that is a reason for some of the price increases we see so often. Perhaps limiting brides to using Disney for all services can allow them to "hold" prices on the packages for more than a year. (Yeah, I know...that's a real stretch!
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I think it came down to copyright issues.
While we all know that professional photographers come to Disney to take pictures, but they don't use them to advertise or promote their business. They are there because they are on vacation with family, or because they like the photo opportunities. Those photos become part of their private collection.
The outside vendors who do Disney weddings are using Disney icons (even if only as a back-drop) to advertise and promote their business and I think that's where the problem comes in.
Disney has always been extremely protective of their copyrights and I think this is just another way to enhance that protection.