Okay, I've given it a little thought. Here's my thought process:
Let's say a potential reward is a pp-up/pp-max, and untradeable to boot. Now, that's a pretty good reward right? If someone can get this for free, why would anyone buy one when they can wait for a free one?
This can be applied to any reward in this suggestion, as I said; it's not a bad idea. It just really has to be thought about. The economy is already pretty bad (in my opinion obviously, i'm not some sort of economy expert).
A reward I can possibly see is a random pokemon that you do not currently own (in its base form), that is classified as a gift pokemon just as a thank you for logging in and continuing to play. Even that has its drawbacks, i'm aware.