"Nothing we've seen from Brad violates any current rules pertaining to the use of social media during races. As such, he won't be penalized. We encourage our drivers to use social media to express themselves as long as they do so without risking their safety or that of others."
Keselowski was tweeting during a red flag, so it wasn't like he was roaring along with one hand on the wheel while the other one tapped away casually on his phone. NASCAR's decision is being hailed for making sense, but the motive driving it is likely that NASCAR sees a chance to make something else.
NASCAR won't penalize Keselowski for using Twitter during Daytona 500 [From the Marbles]