Previously Blizzard has said they wanted to keep it high to stop people from thinking that it was a main part of the game, when in reality it's a side game. They didn't want people to get distracted from the real point -- leveling up your character. While that's not an issue for those of us that have played WoW for a long time, the whole system can seem like a gigantic thing to new folks -- and one that might confuse them as to what they're supposed to be doing.
Stockton's tweet is as follows:
Just hotfixed the cost of Battle Pet Training to 10g. We still don't want newbie players but 100g was obviously overshooting that goal.- Cory Stockton (@mumper) October 5, 2012