On top of their salary, they also get all tournament fees paid, free gas, truck, boat electronics, tackle, clothes, etc, etc, etc. Don't forget that they have numerous sponsors, and are getting paid by more than one company. Also, on top of all that pay, they get all of their winnings too... Also, any profits for making appearances, their personal brands of tackle, fishing shows, etc.
Keep in mind also that only the cream of the crop are making this kind of money. Most of the other pros are just making enough money to keep fishing. While the least paid sports athelete is breaking the bank...
I agree that it is not nearly the pay of other prime sports, but they aren't hurting either... My friend is an FLW pro, and even the he doesn't always make the cut, he still makes enough money to support his family and have a nice lifestyle. He also just finished 10th on Okeechobee which helps a bit too...