And exactly 'how' would you warn that a group of people are going to be attending your park? If Disney said "The Hansen family reunion will be going on during these dates .......", and you took offence at this, you can sue for defamation. At the least, your company (by virtue of acknowleging the gathering) is now shown as supporting and 'sponsoring' the event.
Disney is in the 'family entertainment' business. They know this, but they also have a legal department. They cannot prevent a gathering of customers, they can (and to the best of my knowlege actually do) enforce rules and conduct guidelines for EVERYONE in the park.
Hypothetically, if you own a resturant; and neo-Nazi's show up. If you deny them service, you are discriminating. If you publish, you are going to be associated as condoning or (at worst) supporting that group. What you CAN do, is control the behavior of this group while on your property. But that is about it.
Why not warn people --- even as someone suggested if certain religious groups were having a meeting there that day --- maybe you don't want to be handed the Watchtower if it's Jehovah Witness Day --- but you could decide for yourself and your own family. It's not just Gay Day --- but what if you'd rather not bring your family on Sadists day or whatever else?
Disney gives discounts and blocks off groups of rooms. It is no different than any convention on the disney property. Disney has staff dedicated to facilitating operations for such tour operators.
It is no sweat for disney to post a page saying these operators/groups are being welcomed to the disney parks on these days. No different than a hotel lobby saying which events are in which hall. Not unlike any other convention.
When the FL Bar has a meeting there, I ask the disney reservations and they direct to the proper hotel and dates on the property. It is not a disney event, it is a "group" that is just showing up, but they sure want me to know about it. You don't even have to be a lawyer to ask about it, they will even tell your secretary if she asks.
Disney has no qualms about telling the public they have a bunk of lawyers wandering around but for some reason they have a big problem letting people know a bunch of homosexuals are partying on the property in the same areas as everyone's children.