According to NASCAR:
Following the NASCAR Sprint Cup Series event last Sunday at Phoenix International Raceway, Denny Hamlin made some disparaging remarks about the on-track racing that had taken place that afternoon. While NASCAR gives its competitors ample leeway in voicing their opinions when it comes to a wide range of aspects about the sport, the sanctioning body will not tolerate publicly made comments by its drivers that denigrate the racing product.
Ryan Pemberton, vice president of competition at NASCAR, said that Hamlin’s comments were different from constructive criticism. He called them “damaging.”
After the race, Hamlin said:
I don’t want to be the pessimist, but it did not race as good as our Generation 5 cars. This is more like what the Generation 5 was at the beginning. The teams hadn’t figured out how to get the aero balance right. Right now, you just run single-file and you cannot get around the guy in front of you. [If] you would have placed me in 20th-place with 30 (laps) to go, I would have stayed there — I wouldn’t have moved up. It’s just one of those things where track position is everything.
Hamlin also said that despite starting in the back and finishing third, he did not pass a lot of cars on the racetrack.
While those comments don’t seem to be too outlandish, NASCAR has made it clear that it will not tolerate public negative comments about the racing.
In November of 2011, Brad Keselowski was fined $25,000  for making disparaging comments at a NASCAR Hall of Fame event about NASCAR’s move to fuel injection. At that time, Keselowski said:
We’re not doing this because it’s better for the teams. I don’t think we’re really going to save any gas. It’s a media circus, trying to make you guys happy so you write good stories. It gives them something to promote. We’re always looking for something to promote, but the honest answer is it does nothing for the sport except cost the team owners money. Cars on the street are injected with real electronics, not a throttle body (like in NASCAR). So we’ve managed to go from 50-year-old technology to 35-year-old technology. I don’t see what the big deal is.
In the summer of 2010, Denny Hamlin and Ryan Newman  were also fined for disparaging comments they made about NASCAR.
This time around, however, Hamlin is refusing to pay the fine and he will appeal. On NASCAR Now, Hamlin said:
Ultimately, I’m not OK with it. This is the most upset and angry I’ve been in a really, really long time about anything … anything that relates to NASCAR. The truth is what the truth is. I don’t believe in this. I’m never going to believe in it. As far as I’m concerned, I’m not going to pay the fine. If they suspend me, they suspend me. I don’t care at this point.
Per the NASCAR rulebook, failure to pay a fine can result in suspension, and the money can be deducts from race winnings.
Update: Denny Hamlin has dropped his appeal of the fine. Hamlin tweeted:
After a lot of thought I have decided not to appeal the fine NASCAR has issued. Dragging myself, my team and NASCAR through the mud for the next 2 weeks would not be good for anyone. I firmly believe I am in the right on this issue and will stand behind my decision not to pay. I understand NASCAR will do what they feel is necessary based on my decision. Thanks to all of my fans and peers who have supported me in this decision. I look forward to putting it to rest.
— Denny Hamlin (@dennyhamlin) March 14, 2013