This may be splitting hairs, but technically it's add-on content, not DLC. Which means, it doesn't initially come with any additional cost (similar to the '91 Warriors and '02 Kings that were given to those who pre-ordered or acquired 2K during its first shipment).
Overall, I agree with your sentiment though, I feel something that's a part of the NBA should undoubtedly be a part of NBA2K without necessitating an add-on, DLC, or whatever else.
I'll say, that's a pleasant surprise. With the mode supposedly not being available to all users, I had a difficulty believing it'd be implemented in any other modes. On one hand, it sort of lightens the blow of the dunks being activated by Guitar Hero controls. On the other hand, the fact the dunk contest is implemented into Associations and My Player only further disappoints me that it has horrible controls. As in, it'd be so cool to participate in a mid-season dunk contest, if you were actually controlling what your player was doing.