Its simple, Facebook can't set the precedent that people who exploit bugs in this way get paid. If they did, every Joe who felt that their particular bug wasn't being addressed quite right would think that public exploitation is the faster route to their reward.
Conversely, they might be setting the opposite precendent, that they might ignore your intial email if you don't speak perfectly, and if they ignore your initial email, even hacking Zuck's account won't get you any offical recognition.
The bad precendent is that if you're not a great english speaker, you might as well sell your bug on the black market. This is not good for facebook.
