My guess is that everything is set to go in the backend, but the music industry wants to see how successful Apple Music will be before they sign off on it. Notice how the first, probably large batch of 3-months-free users ended right about the time iOS 9 came out, when we were told the 100,000 track limit would debut. There was a lot of press about people dumping Apple Music when the trial was over. So it makes sense to me that the music industry wants to see how Apple Music performs with paying users on board.Yeah, I'm sure everything was specified in excruciating detail when they signed contracts with the labels, including library limits. Maybe as a result of their negotiations over Apple Music they were able to stretch those limits.
There's no technical reason to stop at 25,000; even storage space isn't a great concern as most tracks are matched with content already in the iTunes library. On Apple's end it is really as simple as changing a value in their code.