Horses will receive the xp for entering the shows at the moment they enter the show, just like the players do.
The top players will probably dislike/hate the change, as it means that you can no longer schedule 400k shows at level 1 and then enter them all.
This change will increase the complexity of the game and I am sure that in time you will learn to like the challenge it brings.
It may not be very obvious right now, but it will take more thinking in scheduling your effort, it will also underline the importance of some things such as skills, specs, tack, tack specialization, bonuses etc.
The aim is to balance the game more, and to move the focus from entering zillions of shows to actually calculating your training steps in order to beat your opponents' horses.