My guess is that it is to stop people stocking up on vendor rep pets. Instead of learning 20 pets and caging them, you will now have to buy only max 3, learn them, and perhaps wait a few days before you can cage any of them, or only cage one unique pet per week, not two of the same type. Well, something like that.
I guess I can just go out and say it, pets like Sporeggar rep, and the Northrend pet named Pengu are rep based and cageable, so people stock up on them and sell. I think it may be a way to reduce the overflow of those pets in the market. At first after the patch these pets were really overpriced, a week ago they were still up around 3000g, but now people have really caught on, and there are tons @ ah for ~200g, which is still a nice profit for those who do it, of course, but it seems to have normalized somewhat as it becomes common knowledge.
...come to think of it, perhaps there is no need for blizzard to fix it after all, it will fix itself, and I am wrong in my guesswork.