Its a bit sad that Disney is that protective about their movies and its characters. Hopefully the success (and the money) that KH3 brought my make them change it a bit. Otherwise it might just be better to have older disney movies in the game, maybe they dont care that much about them then..
If that's the case, it'd be kinda funny because I think those are the ones that fans would want the most, like Treasure Planet.
But then again, we're talking about movies like Frozen here -- probably the biggest Disney (not talking Marvel or Star Wars) movie in recent memory. Like kids knew the songs word for word, Elsa and Anna were popular Halloween costumes, birthday parties had cosplayers show up as the characters... This movie was a monster. So it makes complete sense for big wigs at Disney who probably don't really know or understand Kingdom Hearts to be hesitant with a movie like that. Toy Box seemed perfectly fine, same with Monstropolis and The Caribbean. San Fransokyo turned out really well too. I wish it wasn't like that, but I completely see why Disney would want creative control when it comes to certain worlds and characters.
As for why they're all of a sudden hesitant with summons now, idunno man, that's weird. Like that sounds like an issue you'd have in KH1, not at the most highly anticipated saga finale that's bound to break sales records. Especially when Simba's been a summon in the first game and we went to his world in the second numbered title.
It goes to show though that there's a lot of loose ends that needs to be dealt with when making a Kingdom Hearts game, so when people were complaining about the lack of a release date, this is probably a part of why things get held up. I hope for the next title, Disney loosens their grip, because I'm sure we could see some real cool stuff happen once they do.