1. The NFL’s: the ball can’t be used as a prop, hence, dunking it like a basketball is a no-no. Although, I’m willing to bet that 1% of the reasoning behind this rule is that the NFL doesn’t want to you to think about any other sports while watching their precious FOOTBALL.
2. Everyone else’s: The NFL just hates fun, and goalpost dunks are way, way too much fun.
Here’s a solution: allow goalpost dunks, but if the player gets rim-checked and doesn’t get the ball over the crossbar, or doesn’t actually throw it down over the crossbar (think Malcolm Smith in the Super Bowl), it’s a 15-yard penalty. That’ll make some guys think twice about trying it.