They will still have Christmas shows during the day as in the past (confirmed on the Facebook page). I can see why the uproar over it being a separate ticketed event. However, the park normally closed at 6 or 7 in December if I remember correctly (with the exception of New Years Eve). The only real difference I can see to normal daytime operations would be to close perhaps around 5 to get everything ready. I'm guessing they need to see if the event will be profitable and draw a crowd. Perhaps they include it with admission in future years (hopefully). I love the event at SWO and have wanted BGT to get one similar.
The thing with Seaworld; they generally are busier than BGT in December simply due to the fact of it's location and closer competition, so it makes sense to have it included there. Also, BGW is normally not open in December, and open in the afternoon for this event only.